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The EU is preparing a “plan B” on how to keep economic sanctions against Russia should the Trump administration abandon Ukraine peace talks and seek rapprochement with Moscow, according to the bloc’s top diplomat.

US President Donald Trump had vowed to bring a swift end to Russia’s more than three-year-long war against Ukraine, but has failed to force a peace agreement during his first 100 days in office as Moscow and Kyiv both reject elements of his administration’s proposals.

“It’s a question whether the Americans will want to leave,” Kaja Kallas, the EU’s high representative for foreign and security policy, told the Financial Times. “We see signs that they are contemplating whether they should leave Ukraine and not try to get a deal with the Russians because it’s hard.”

A spokesperson for US secretary of state Marco Rubio said on Tuesday: “If there is not progress, we will step back as mediators in this process.”

Trump’s suggestions for economic rapprochement with Moscow as part of a peace deal have fuelled concerns that some EU countries will demand that Brussels also moves to lift EU sanctions against Russia in coming months.

European officials are also worried about the consequences of the US potentially allowing its companies to restart economic co-operation with Russia while EU business are still barred from doing so.

Kallas said there was a “plan B” to maintain economic pressure on Russia should Hungary block the rollover of EU economic sanctions in July, but stressed that Brussels was still focused on keeping all member states aligned.

“There is also a plan B but we have to work for plan A; because otherwise you concentrate on plan B and then that will happen,” she said, adding that talks were ongoing with Washington and other international partners to ensure that the western sanctions regime was kept in place.

The FT has previously reported that one option is for national governments to adopt the sanctions individually to circumvent a Hungarian veto, such as Belgium issuing a royal decree to keep €190bn worth of Russian state assets frozen on Belgian soil.

“Belgium will not do something nationally,” a senior Belgian official told the FT.

But Kallas did acknowledge that there are discussions in some EU capitals on whether to follow the Trump administration if it abandons Ukraine and resets relations with Russia.

“It is clear that these types of discussions are going on in certain member states and maybe hopes that we don’t really have to support [Ukraine] any more,” said Kallas, a former Estonian prime minister. “But it’s also a false hope, because if you look at Russia that is investing more than 9 per cent of its GDP on the military, they will want to use it again.”

As part of its peace proposals, the US has offered to lift sanctions against Moscow — imposed in conjunction with the EU — and restart economic co-operation with Russia in sectors including energy. Hungary has threatened to veto a necessary unanimous vote at the end of July to extend the EU’s sanctions regime.

Kallas said Europe could step in financially to help Ukraine in case of a US withdrawal, but acknowledged that “in terms of military support, of course it’s harder to fill the gap if the Americans are leaving”.

She said Brussels and other European capitals were focused on “still working with the Americans and trying to convince them why the outcome of this war is also in their interest, that Russia doesn’t really get everything that it wants”.

But Kallas said that no EU country would accept recognition of Crimea as Russia, an element of the US proposal that is a major red line for Kyiv.

“I can’t see that we are accepting these kind of things. But we can’t speak for America, of course, and what they will do,” she said. “On the European side, we have said this over and over again . . . Crimea is Ukraine.”

“There are tools in the Americans’ hands that they can use to put the pressure on Russia to really stop this war,” Kallas added. “President Trump has said that he wants the killing to stop. He should put the pressure on the one who is doing the killing.”