European leaders closed ranks on Sunday in support of Kyiv at a London summit, where they pledged to spend more on security and assemble a coalition to defend any truce in Ukraine.

And French President Emmanuel Macron, flying back from the summit, said in a newspaper interview that France and Britain wanted to propose a partial one-month truce.

The talks, which brought together 18 allies, came just two days after US President Donald Trump berated his Ukrainian counterpart Volodymyr Zelensky in front of reporters at the White House.

UK Prime Minister Keir Starmer said that Britain, France "and others" would work with Ukraine on a plan to stop the fighting, which they would then put to Washington.

The London meeting came at a delicate moment for war-battered Ukraine, facing uncertain US backing and on the back foot against Russia's three-year war.

Trump's row with Zelensky also raised fresh questions over the US commitment to Nato.

Starmer said Europe found itself "at a crossroads in history".

"This is not a moment for more talk - it's time to act. It's time to step up and lead and unite around a new plan for a just and enduring peace."

With no guarantee of US involvement, "Europe must do the heavy lifting", Starmer said.

Several countries were ready to help defend any truce, he added - without naming them.

Macron, in an interview with France's Le Figaro newspaper, said he and Starmer were working on a one-month truce in Ukraine, "in the air, at sea and on energy infrastructure".

Zelensky was warmly embraced by many of the world leaders at the summit.

Outside the UK leader's Downing Street home, people gathered to show their support for Ukraine, some dressed in the country's blue and yellow national colours. (AFP)