Ukraine and European leaders agreed on Saturday to an unconditional 30-day ceasefire on May 12 with the backing of US President Donald Trump, threatening President Vladimir Putin with new “massive” sanctions if he failed to comply.

The announcement was made by the leaders of Britain, France, Germany, Poland and Ukraine after a meeting in Kyiv, after which they held a phone call with Trump. The US leader, who wants a rapid peace, has not commented publicly on the course of action.

“So all of us here together with the US are calling Putin out. If he is serious about peace, then he has a chance to show it,” British Prime Minister Keir Starmer told reporters at a news conference.

“No more ifs and buts, no more conditions and delays.”

Following through on the threat would be a dramatic sign of growing Western unity after months of unpredictable US policy that has seen Trump tear up the approach of his predecessor since entering the White House in January.

After engaging directly with Russian officials, clashing publicly with Ukrainian President Volodymyr Zelensky and briefly cutting vital military aid to Ukraine, the Trump administration has patched up ties with Kyiv and signed an arduously-negotiated mineral resources deal.