"You set an example to the whole world when it comes to genuine courage," Biden told Zelensky in Vilnius, where they attended the NATO summit. "Not only all of you but your people -- your sons, your daughters, your husbands, your wives, your friends: you're incredible."

Biden said that Russia's military, which launched its devastating invasion of swaths of Ukraine last year, was acting "like something out of the 14th century."

Following tensions at the Vilnius summit over NATO's rejection of Ukrainian pressure to be given accelerated membership in the alliance, Biden acknowledged the "frustration" sometimes felt by Zelensky.

"But I promise you: the United States is doing everything we can to get you what you need as rapidly as we can get it to you," Biden said.

Biden highlighted commitments announced Wednesday by the G7 group of wealthy democracies to provide Ukraine extended military and economic support while its NATO bid is held up by the war against Russian invasion.

"The bad news is we're not going anywhere. You're stuck with us," Biden joked.

Zelensky, who caused a stir on Tuesday with an angry tweet criticizing NATO's refusal to set a clear membership timeline, went out of his way at the meeting with Biden to express gratitude for the massive US support during the war.

"It's huge support. I understand that it's all your money. But you have to know that you spend this money not just for fighting, you spend this money for our lives and I think that we save the lives for Europe and for all the world," Zelensky said, speaking in occasionally broken English.

"Thank you very much that all these days you are with us. I mean, you and Americans, all of Americans, and Congress... (were) shoulder to shoulder with us from the first days of full-scale war. We appreciate for this very much."

"Thank you for acknowledging the American people," Biden responded, noting that the United States had given far more than any other Western nation in weapons and funds to support Ukraine.

Signalling the drop in tensions, Biden answered in place of Zelensky when a reporter asked how long it should be before Ukraine was admitted to NATO, joking: "An hour and 20 minutes."

Washington says Ukraine can't enter NATO while it is at war, because that would trigger the alliance's collective defense clause and immediate war between the United States and Russia.