“The reasons are still unknown, as well as their current well-being,” Andrii Sybiha wrote on X. “We have already sent an official note demanding an immediate release of our citizens.”

According to Ukraine’s state savings bank, Oschadbank, the seven workers were in two vans carrying $80m (£60m) worth of cash and 9kg of gold in a regular transport between Austria and Ukraine. They were “unjustifiably detained” and GPS data showed their vehicles in Budapest, it said.

Hungary’s tax authority said on Friday that it was conducting criminal proceedings and added that one of the group was a former general of Ukraine’s intelligence service.

“This year alone, more than $900m, €420m and 146 kilograms of gold bars were transported to Ukraine through the territory of Hungary,” the national tax and customs administration said in a statement.

Relations between Ukraine and Hungary have deteriorated during Russia’s full-scale invasion of Ukraine and have descended into a war of words over a halt to Russian oil supplies through the Druzhba pipeline in Ukraine.

Prime Minister Viktor Orbán made no mention of the seven bank workers in his regular radio appearance on Friday, although he did say “transit shipments” important to Ukraine would be halted until a row over Russian oil supplies was sorted out.

Hungarian reports describe how black-clad officials from Hungary’s TEK counter-terrorism centre raided the Ukrainian-registered vehicles on Thursday and then their convoy headed to Budapest.

Responding to the incident, Ukraine’s foreign minister accused Hungary of “state terrorism and racketeering”.

“We are talking about Hungary taking hostages and stealing money,” said Sybiha on X.

Orbán, seen as Russia’s closest ally in the EU, has accused Ukraine of deliberately halting Russian oil through the pipeline. Kyiv says the pipeline was damaged in a Russian air strike in January and Orban has threatened to “force the Ukrainians to restart deliveries”.

The Hungarian leader, whose party is trailing in the polls ahead of next month’s pivotal elections, has also blocked a €90bn (£78bn) EU aid package seen as vital for Ukraine’s financing in a bid to force the resumption of oil supplies.

Orbán has repeatedly opposed EU funding for Ukraine, arguing it prolongs the war. He has focused much of his election campaign on an anti-Ukraine message.

Last month he ordered increased security for Hungary’s energy infrastructure, making unsubstantiated claims that Kyiv was preparing “further actions to disrupt” the energy system.

Ukrainian President Volodymyr Zelensky has criticised Orbán for blocking the aid package and said on Thursday that the Druzhba pipeline might only become operational again in another four to six weeks.

“To be honest, I would not restore it. This is my position. I expressed it to European leaders… because this is Russian oil,” he told a briefing. “Russians are killing Ukrainians and we have to give oil to Orbán, because he, the poor thing, cannot win the election without this oil.”

The Ukrainian leader also hinted that if Hungary’s prime minister continued to block EU funding for Ukraine, then Ukrainian armed forces would be given his address to “communicate with him in their own language”.

Orbán has said Budapest will use “political and financial tools” to force Ukraine to reopen the pipeline carrying Russian oil to Hungarian refineries.

Druzhba is the main route for delivering Russian oil to Hungary and Slovakia and shipments of Russian oil to both countries have been cut off since 27 January.

In a separate development this week, Hungarian Foreign Minister Peter Szijjarto travelled to Moscow and secured the release of two ethnic Hungarian prisoners of war.

Hungary says the men, who are from Ukraine’s Transcarpathia region, have dual Hungarian and Ukrainian citizenship and has accused Kyiv of conscripting them into the army.

Kyiv has described their transfer to Hungary as a “gross violation of international humanitarian law”.

It is a rare move between Nato allies, and symbolic of the depth to which US-Hungarian relations have sunk since Russia’s full-scale invasion of Ukraine almost three years ago.

“Antal Rogan is a primary architect, implementer and beneficiary of this system of corruption,” read the statement, made by outgoing US Ambassador David Pressman.

Pressman leaves Budapest next week, after two and a half years spent as an unusually active diplomat, travelling the country and frequently criticising the Orban government.

His departure comes days ahead of Donald Trump’s return to the White House, and the president-elect has a far more positive view of Viktor Orban than the Biden administration, seeing him as a close political ally.

“While Minister Rogan’s media megaphones will try to make this a story about partisan politics or an affront to sovereignty, today’s decision is actually the reverse,” Pressman told reporters in Budapest on Tuesday.

“It is not the United States that threatens Hungary’s sovereignty, but rather the kleptocratic ecosystem Minister Rogan has helped to build and direct and that he has benefited from personally.”

The ambassador’s statement was immediately attacked by Hungarian Foreign Minister Peter Szijjarto.

“This is the personal revenge of the ambassador who was sent to Hungary by the failed US administration, but left without success and in disgrace,” Szijjarto wrote on Facebook.

“How good it is that in a few days’ time the United States will be led by people who see our country as a friend and not as an enemy.”

A former US ambassador to Hungary, David Cornstein, also came to Rogan’s defence: “The move by outgoing ambassador David Pressman is an example of the current US administration’s hostile stance towards Hungary, right down to the last hour.”

The question for the incoming Trump presidency, and its next ambassador to Budapest, is whether they will immediately overturn the sanctions against Antal Rogan.

The answer is not as obvious as might appear.

Rogan also oversees the domestic secret services, and there have been indications from several Nato countries that Hungary is no longer trusted with sensitive information because of the Orban government’s close relations with Russia’s Vladimir Putin.

And for all the expressions of outrage at the decision to impose sanctions on Orban’s head of cabinet, several senior figures in the Fidesz establishment have long been privately upset by the lifestyle of Rogan and others, by the power he wields, and the distance from the conservative and Christian values that the party proclaims so loudly.

Orban, who has maintained closer ties with Putin than other European countries throughout Russia’s war in Ukraine, said on X that the pair’s phone call on Wednesday lasted an hour and that “these are the most dangerous weeks” of the war.

“No one should boost [their] personal image at the expense of unity,” Zelenskyy said on X.“We all hope that Orban at least won’t call Assad in Moscow to listen to his hour-long lectures as well,” he added, referring to Russia’s decision to grant deposed Syrian President Bashar al-Assad political asylum.

Kyiv has repeatedly called for unity among its allies on isolating Putin and has said any discussions on the war must involve Ukraine.

Ukrainian officials have also warned that a ceasefire in the 33-month-old war would benefit Moscow, as it could freeze current front lines with Russia holding about 20 percent of Ukrainian territory.

The Kremlin said Putin had told Orban during the phone call that Kyiv’s stance excluded the possibility of a peaceful settlement to the war.

The call was initiated at the Hungarian leader’s request, the Kremlin said, and came a day after Hungary’s top diplomat said his country would forge ahead with its self-styled Ukraine “peace mission”.

Orban responded to Zelenskyy’s gripe on X by saying that Hungary had proposed a “Christmas ceasefire” and a prisoner exchange but that the Ukrainian leader had “clearly rejected and ruled this out”.

Orban did not offer more details about exactly when or how he had proposed such a truce, or whether the same offer was made to Putin.

Ukraine, in turn, denied that Orban discussed a Christmas truce with them.

“As always, the Hungarian side did not discuss anything with Ukraine. As always, the Hungarian side did not warn [us] about its contacts with Moscow,” presidential aide Dmytro Lytvyn said in a statement.

“There can be no discussions about the war that Russia wages against Ukraine without Ukraine,” Zelenskyy said on Wednesday.

Orban has repeatedly called for peace talks and refused to send military aid to Ukraine since Russia launched its offensive in February 2022.

He angered fellow EU leaders in July by engaging in diplomatic talks with Russia on ending the conflict, just days after Hungary took over the bloc’s rotating six-month presidency.