The UK prime minister indicated some EU nations could be prepared to increase defence spending to protect any peace deal that is agreed between Ukraine and Russia, SKY News reported.
But speaking at a news conference following the summit of EU leaders in central London, Sir Keir acknowledged that no such coalition had yet been formed - and warned that "not every nation will feel able to contribute".
Instead, he said "those willing" - though he did not state which countries this included - would "intensify planning now with real urgency".
In a sign this could see EU member states send their own troops to Ukraine, he added: "The UK is prepared to back this with boots on the ground and planes in the air, together with others. Europe must do the heavy lifting."
The summit of EU leaders, along with Canada and Turkey, had been planned for over a week but took on an added urgency following the disastrous meeting between President Trump and Volodymyr Zelenskyy at the White House on Friday.
In front of the world's cameras, Mr Trump accused the Ukrainian president - who is met with the King at Sandringham - of "disrespecting" the United States and "gambling with World War Three" when he expressed scepticism that Vladimir Putin could be trusted to honour ceasefire agreements.
MNA
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