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

European leaders have called US President Donald Trump to discuss proposals for a 30-day ceasefire between Russia and Ukraine from Monday while on a visit to Kyiv. The call came after leaders of the so-called "coalition of the willing" held a meeting to discuss advancing peace talks.

The ultimatum follows a proposal by the United States in March for a 30-day ceasefire, which Ukraine accepted. However, Russia has thus far declined, instead insisting on conditions that have been deemed unacceptable by Ukraine and its allies.