“There have been significant hostilities in the last two weeks, before a ceasefire was brokered,” Geir Pedersen told the UN Security Council in New York on Tuesday, warning that a military escalation could be “catastrophic”.

The remarks come after the Syrian National Army (SNA), a group of fighters backed by Turkiye, has battled with the Kurdish-led Syrian Democratic Forces (SDF) in the wake of al-Assad’s removal.

The SDF is the main ally in a United States-led coalition against ISIS (ISIL). It is spearheaded by the People’s Protection Units (YPG), which Ankara sees as an extension of the Kurdistan Workers’ Party (PKK) armed group that it outlaws and that has fought the Turkish state for 40 years.

Last week, SNA fighters seized the northern city of Manbij from the SDF, which then headed east of the Euphrates River as a US-mediated ceasefire came into effect.

Shortly after Pederson addressed the UN Security Council, a US State Department spokesman said the ceasefire in northern Syria had been extended until the end of this week.

We continue to engage with the SDF, with Turkey about a path forward,” State Department spokesperson Matthew Miller said, adding it was not in the interest of any party to see increased conflict in Syria.

“We don’t want to see any party take advantage of the current unstable situation to advance their own narrow interests at the expense of the broader Syrian national interest.”

Pedersen also called on the UN’s Security Council to ensure that Israel “cease all settlement activity in the occupied Syrian Golan”.

Pedersen noted Israel has conducted more than 350 strikes on Syria following the departure of the former regime, including a major strike on Tartous.

“Such attacks place a battered civilian population at further risk and undermine the prospects of an orderly political transition,” he said.

The UN envoy warned against plans announced by Israel’s cabinet to expand settlements inside the Golan, occupied by Israel since 1967 and annexed in 1981.

On Tuesday, Israeli Prime Minister Benjamin Netanyahu held a security briefing inside the UN-patrolled buffer zone in the Golan Heights that Israel seized earlier this month.

“Israel must cease all settlement activities in the occupied Syrian Golan, which are illegal. Attacks on Syria’s sovereignty and territorial integrity must stop,” said Pedersen.

Rob Geist Pinfold, a professor of peace and security at Durham University, told Al Jazeera that “Israel is looking to launch as many strikes [as possible] while it still has the freedom to do so.”

He suggested that “Israel is adding to the instability [due to their] fear power vacuums and ungoverned spaces on their borders.”

Geist Pinfold also said that the “international community’s response has certainly been lacklustre so far.”

Pederson also called for “broad support” to end the Western-backed sanctions imposed on Syria in order to allow for reconstruction in the war-ravaged country.

“Concrete movement on an inclusive political transition will be key in ensuring Syria receives the economic support it needs,” Pedersen said.

“The needs are immense and could only be addressed with broad support, including a smooth end to sanctions, appropriate action on designations, too, and full reconstruction.”

Western countries are wrestling with their approach to Hayat Tahrir al-Sham (HTS), which spearheaded the takeover of Damascus, and is a former al-Qaeda affiliate.

Despite moderating its rhetoric in recent months, HTS has been designated as a “terrorist” group by many Western governments.

Multiple Middle Eastern nations and Israel’s ally Germany on Monday denounced Israel’s decision to double the Israeli settler population in the illegally occupied Syrian territory.

The criticism comes amid a flurry of diplomatic activity in Syria after the ouster of al-Assad as the new caretaker government takes over.

Israel approved the settlement expansion days after the Israeli military seized more Syrian territory after al-Assad’s fall.

Syria has also been under intense Israeli bombardment with hundreds of attacks targeting military sites and research centres across the country.

Several countries have denounced Israel’s assaults on Syria. This week, more countries criticised Israel’s plans for the Golan:

• Qatar rebuked the scheme as a “new episode in a series of Israeli aggressions on Syrian territories”.
• Jordan called it a “blatant violation of international law”.
• Turkiye denounced the move as a bid by Israel to “expand its borders”.
• Saudi Arabia slammed “continued sabotage of Syria’s chances of restoring its security and stability”.
• Egypt condemned the plans as “a flagrant violation of Syria’s sovereignty and territorial integrity”.
• Germany said it is “perfectly clear under international law that this area controlled by Israel belongs to Syria”.

Israel occupied the Golan in 1967 and subsequently annexed it in 1981 in a move that is seen as illegitimate by most of the international community.

The recent Israeli plan will allocate more than 40 million shekels ($11m) to increase the settler population.

“Strengthening the Golan is strengthening the State of Israel, and it is especially important at this time. We will continue to hold onto it, cause it to blossom and settle in it,” Israeli Prime Minister Benjamin Netanyahu said on Sunday.

The new authorities in Syria have signalled that they do not seek confrontation with Israel.

Meanwhile, the United States said on Monday that its forces have conducted air strikes in Syria that killed 12 ISIL (ISIS) “terrorists”.

“The strikes against the ISIS leaders, operatives, and camps were conducted as part of the ongoing mission to disrupt, degrade, and defeat ISIS, preventing the terrorist group from conducting external operations and to ensure that ISIS does not seek opportunities to reconstitute in central Syria,” the US military said in a statement.

The US attacks came as the European Union’s top foreign policy official said the bloc’s envoy to Syria has begun high-level talks with the country’s new leaders and the EU will organise a fundraising conference to aid Syria’s transition.

Vice-President of the European Commission, Kaja Kallas also told reporters after a meeting with European foreign ministers that Iran and Russia should have “no place” in Syria’s future.

“Many foreign ministers emphasised that it should be a condition for the new leadership to eliminate Russian influence [in Syria],” Kallas said.

Russia, which backed al-Assad with weapons and air strikes during the war, has military bases in Syria, whose future remains unclear.

Earlier on Monday, al-Assad issued his first statement since he was deposed, saying the country is “in the hands of terrorism”.

Opposition forces, led by Hayat Tahrir al-Sham (HTS), launched a blistering offensive in November that toppled the former Syrian president on December 8.

The Syrian war started as a largely unarmed, peaceful uprising against al-Assad in March 2011 but morphed into a full-blown war, which killed hundreds of thousands of people and turned millions into refugees.

Prime Minister Benjamin Netanyahu said the move was necessary because a “new front” had opened up on Israel’s border with Syria after the fall of the Assad regime to an Islamist-led rebel alliance.

Netanyahu said he wanted to double the population of the Golan Heights, which Israel seized during the 1967 Six-Day War and is considered illegally occupied under international law.

Israeli forces moved into a buffer zone separating the Golan Heights from Syria in the days following Assad’s departure, saying the change of control in Damascus meant ceasefire arrangements had “collapsed”.

There are more than 30 Israeli settlements in the Golan Heights, which are home to an estimated 20,000 people. They are considered illegal under international law, which Israel disputes.

The settlers live alongside some 20,000 Syrians, most of them Druze Arabs who did not flee when the area came under Israeli control.

Netanyahu said Israel would “continue to hold on to [the territory], make it flourish and settle it”.

The announcement comes a day after Syria’s new de-facto leader Ahmed al-Sharaa criticised Israel for its ongoing strikes on military targets in the country, which have reportedly targeted military facilities.

The UK-based Syrian Observatory for Human Rights (SOHR) has documented more than 450 Israeli air strikes in Syria since 8 December, including 75 since Saturday evening.

Al-Sharaa – also known as Abu Mohammed al-Jolani – said the strikes “crossed red lines” and risked escalating tensions in the region, though he said Syria was not seeking a conflict with any neighbouring state.

Speaking to Syria TV, which was seen as pro-opposition during the civil war, al-Sharaa said the country’s “war-weary condition, after years of conflict and war, does not allow for new confrontations”, Reuters reported.

The Israel Defense Forces (IDF) has not commented on his remarks, but previously said the strikes were necessary to stop weapons falling “into the hands of extremists”.

President Bashar al-Assad and his family fled to Russia and took up asylum when al-Sharaa’s Islamist group Hayat Tahrir al-Sham (HTS) led other rebel factions in a lightning offensive on Damascus.

The groups are continuing to form a transitional government in Syria, of which al-Sharaa is the theoretical head.

On Saturday, the US Secretary of State Antony Blinken said Washington had made direct contact with HTS, which the US and other Western governments still designates as a terrorist organisation.

United Nations’ Syria envoy Geir Pedersen said on Sunday he hoped for a swift end to sanctions on the country to help facilitate an economic recovery.

“We will hopefully see a quick end to sanctions so that we can see really rallying around building up Syria,” Pedersen said as he arrived in Damascus to meet Syria’s caretaker government and other officials.

Elsewhere, Turkey’s Defence Minister Yasar Guler said Ankara was ready to provide military support to Syria’s new government.

“It is necessary to see what the new administration will do. We think it is necessary to give them a chance,” Guler said of HTS, according to state news agency Anadolu and other Turkish media outlets.

Netanyahu said on Sunday that the decades-old agreement had collapsed and that Syrian soldiers had abandoned their positions, necessitating the Israeli takeover.

“We will not allow any hostile force to establish itself on our border,” he said.Israel captured the Golan Heights in the 1967 war and annexed it. The international community, except for the United States, views it as occupied Syrian territory.

Agricultural areas in the Israeli-occupied Golan Heights were declared closed military zones and some schools shifted to online classes in anticipation of unrest.

Syrians poured into the streets echoing with celebratory gunfire on Sunday after a stunning opposition advance reached the capital of Damascus, putting an end to the al-Assad family’s 50 years of iron rule but raising questions about the future of the country and the wider region.

Joyful crowds gathered in central squares in Damascus, waving the Syrian revolutionary flag in scenes that recalled the early days of the Arab Spring uprising, before a brutal crackdown and the rise of an armed uprising plunged the country into a nearly 14-year war.

Netanyahu hailed the removal of al-Assad on Sunday as an “historic day” that followed the blows delivered by Israel against al-Assad’s supporters Iran and Hezbollah in its recent war on Lebanon.

Meanwhile, Israeli media said on Sunday that the Israeli air force has bombed weapons depots in southern Syria and Damascus to prevent opposition groups from seizing them.

“We attacked ammunition depots in southern Syria and in the Damascus airport area for fears they might fall into the hands of armed groups and local factions,” the Israeli public broadcaster KAN quoted an unnamed Israeli security official as saying.

“Israel is working to thwart any potential threats and prevent any damage to its air superiority in Syria,” the official added.Israeli daily Yedioth Ahronoth said weapons depots and surface-to-surface missile stockpiles were the targets of Israeli attacks in Syria, without providing further details.

Israel has frequently targeted weapons shipments and military installations in Syria throughout the country’s war, citing concerns over the possible transfer of advanced weaponry to hostile groups, including Hezbollah and Iran-backed militias.

An Associated Press journalist in Damascus reported air attacks in the area of the Mazzeh military airport, southwest of the capital on Sunday.

The airport has previously been hit in Israeli air attacks, but it was not immediately clear who launched Sunday’s attack.

The Israeli military refused to comment on the reports and Israel often does not publicly claim responsibility for attacks in Syria.