The British prime minister had said earlier that the UK was prepared to send troops to Ukraine after the war as part of a peacekeeping force but only if the US, Nato’s leading member, provided a “backstop”.
Asked if the US would aid British peacekeepers if they were attacked by Russia, Trump said: “The British have incredible soldiers, incredible military and they can take care of themselves. But if they need help, I’ll always be with the British, okay?”
Nato’s Article 5 holds that Nato members will come to the defence of an ally which comes under attack.
Praising Trump’s “personal commitment to bring peace” in Ukraine, Sir Keir said the UK was “ready to put boots on the ground and planes in the air to support a deal”.
“We’re focused now on bringing an enduring end to the barbaric war in Ukraine,” he said.
But, he added, it must not be a peace deal “that rewards the aggressor or that gives encouragement to regimes like Iran”.
Asked whether Vladimir Putin was trustworthy, the UK prime minister said his views on the Russian president were well-known.
Asked in turn why he seemed to trust Putin and Sir Keir did not, Trump said: “I know a lot of people that you would say no chance that they would ever deceive you, and they are the worst people in the world.
“I know others that you would guarantee they would deceive you, and you know what, they’re 100% honourable, so you never know what you’re getting.”
EU foreign policy chief Kaja Kallas, who had been due to meet US Secretary of State Marco Rubio in Washington before he cancelled the talks “due to scheduling issues”, told BBC News that Putin and Russia did “not want to have peace”.
“For any peace agreement to function, it needs the Europeans as well as Ukrainians on board,” she added.