It follows Donald Trump saying Russia and Ukraine will "immediately" start negotiating, after a two-hour phone call with Vladimir Putin.

The ultimatum from Berlin comes as Ukraine said Russia was still carrying out attacks along the front line, despite a ceasefire proposal agreed on the weekend among Kyiv's allies for a 30-day ceasefire to start Monday. The Kremlin said such an ultimatum "is unacceptable for Russia."

Ukraine’s Western allies including the US are threatening to slap Russia with more sanctions if Moscow fails to sign up to the 30-day truce in Ukraine proposed by the United States.

US Secretary of State Marco Rubio has said he expected Russia to present a Ukraine ceasefire outline within days that will show if it is serious, as Ukrainian President Volodymyr Zelensky accused Russia of buying time.

European leaders threatened new sanctions unless Moscow accepted a cease-fire right away. President Trump seemed to endorse the idea of a meeting between Ukraine’s and Russia’s presidents, and said he might take part.

Trump has shifted U.S. rhetoric from supporting Ukraine toward accepting some of Moscow's narrative about the war. Kyiv and its European allies are working hard to persuade him that it is now Moscow that is holding up a truce.

Delegations from Russia and Ukraine are set to meet for peace talks in Turkey on Friday. But, Presidents Vladimir Putin and Volodymyr Zelensky, nor US President Donald Trump, will be there following days of confusion.

Britain and the European Union have imposed sweeping sanctions targeting Russia, particularly its shadow fleet of oil tankers, over its war in Ukraine.