(Reuters) - Turkey is ready to undertake the duty to observe a possible ceasefire in Ukraine, its foreign minister said during a "coalition of the willing" call with Kyiv's partners on Saturday, a Turkish foreign ministry source said.

The leaders of Britain, France, Germany, Poland -the so-called "coalition of the willing"- and Ukraine have met in Kyiv on Saturday. They agreed to an unconditional 30-day ceasefire on May 12 with the backing of U.S. President Donald Trump, threatening President Vladimir Putin with new "massive" sanctions if he failed to comply.