The president of Finland is urging President Trump to impose a deadline on Vladimir Putin of April 20 to agree a ceasefire in Ukraine.

Speaking to Fox News in London following a weekend visit with Trump in Florida, Alexander Stubb praised Trump's negotiating efforts, saying Trump is "probably the only person in the world who can mediate the peace."

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But he argued the ceasefire negotiation process should not be open ended.

"We need a ceasefire, and we need a date for the ceasefire," Stubb said. "And that date should be the 20th of April."

April 20 would mark three months since Trump's inauguration, and is also Orthodox Easter.

"If President Putin — who is the only one who is not accepting a ceasefire, because the Americans want it, the Europeans want it, the Ukrainians want it — if he doesn't oblige by the ceasefire, then we should go for a colossal set of sanctions coming from the United States and Europe," Stubb said.

Trump has spoken of a "psychological deadline" for Russia to agree to a ceasefire, but has declined to name a date.

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Stubb said Putin "respects, and in many ways fears, Donald Trump."

Finland — a neighbor of Russia's, with a shared border running more than 800 miles — upended decades of neutrality two years ago when it joined NATO, alarmed by the war in Ukraine.

Stubb believes Ukraine should also be allowed to join the military alliance "in the long run" — a position that runs counter to the Trump administration's.

Following talks and a round of golf with Trump in Florida, the Finnish leader said European leaders are heeding American complaints that Europe does not spend enough money on defense, relying instead on the United States.

"Europe needs to take more responsibility for its own security, more responsibility for its own defense," Stubb said. "I think we're doing exactly that."

He described the U.S.-European relationship as "in a transition," but insisted: "We're allies.

"Just because ideologically there are differences at times between Europeans and Americans doesn't mean that we're going to sever or divorce."