Finland's President Alexander Stubb said on Sunday he had told U.S. President Donald Trump that a deadline needs to be set for establishing a Ukraine ceasefire in order to make it happen.

"April 20 would be a good time for a full ceasefire without any conditions ... Because a deadline is needed, because it is Easter and because President Donald Trump will have been in office for three months," Stubb told Finnish reporters in London.

Stubb on Saturday made a surprise trip to meet Trump in Florida, where they discussed strengthening their countries' bilateral partnership and also played a round of golf, Stubb's office said.

Since taking office in January, Trump has focused heavily on ending what he calls a "ridiculous" war in Ukraine. Earlier this week the United States announced separate agreements with Ukraine and Russia to pause their strikes in the Black Sea and against each other's energy targets.