Top U.S. and European military officials discussed Tuesday the mechanics of a possible Ukraine peace deal, a day after President Donald Trump ruled out American boots on the ground but suggested providing air support.

In a flurry of diplomacy aimed at ending the war, Trump brought Ukrainian President Volodymyr Zelenskyy and European leaders to the White House on Monday, three days after his landmark encounter with Russia's Vladimir Putin in Alaska.

But while Trump said Putin had agreed to meet Zelenskyy and accept some Western security guarantees for Ukraine, those promises have been met with extreme caution by Kyiv and Western capitals, and many details remain vague.

Putin proposed holding the summit with Zelenskyy in Moscow, three sources familiar with a call between Trump and the Russian leader told AFP. One source said Zelenskyy immediately said no to meeting in the capital of his country's invader.

The U.S. president nevertheless said he remained "optimistic" about the process after having met separately with Putin and Zelenskyy.