'Take practical steps'
Macron has described Trump's return for a second term in the White House as an "electroshock" and there are initial signs some of his counterparts are being stung into action.
Britain's Starmer, aware of the importance of London showing commitment to European security after Brexit, said Sunday that he was willing to put "our own troops on the ground if necessary" in response to what he called "a once-in-a-generation moment for the collective security of our continent".
Polish Prime Minister Donald Tusk, another key participant, said on Monday he would urge European leaders at the emergency summit to "immediately" boost Europe's defences, warning they do not match Russia's.
"We will not be able to effectively help Ukraine if we do not immediately take practical steps regarding our own defence capabilities," Tusk told reporters.
Trump's special envoy to Ukraine, Keith Kellogg, has said Europe would not be directly involved in talks on Ukraine, though it would still have "input".
French Foreign Minister Jean-Noel Barrot said Sunday it would fall to Europe to guarantee any peace deal in Ukraine, adding he expected the United States to "revise their level of commitment to NATO, including in terms of geography".
The American policy shift "requires that we truly wake up, and even take a leap forward, to take our place for the security of the European continent", Barrot said.