Biden departed Dover Air Force Base in Delaware and was due to arrive in key US ally Britain late Sunday.

On Monday, he meets with the British monarch at Windsor Castle, one of the royal residences, for the first time since Charles III's coronation. The US president did not attend the ceremony, sending First Lady Jill Biden instead.

Their talks are expected to focus on environmental issues, the White House said. Biden will also be meeting with Prime Minister Rishi Sunak at 10 Downing Street.

The main part of Biden's Europe trip will be the NATO summit in the Lithuanian capital Tuesday and Wednesday, where the Western allies will discuss helping Ukraine to oust Russian occupation forces.

Ukraine is pressing for admission to the military alliance but Biden said in an interview aired Sunday with CNN that this cannot happen until the war is over.

Bringing Ukraine in now would mean NATO is at war with Russia, Biden said.

Under its Article 5, NATO is committed to defending any member that comes under attack. "It's a commitment that we've all made no matter what. If the war is going on, then we're all in war. We're at war with Russia, if that were the case," Biden said.

Biden hopes to use the summit to pressure Turkey into dropping opposition to Sweden's all-but-cleared NATO membership bid. Entry requires unanimous consent from all the other members.

In the interview with CNN, Biden also suggested he was eyeing the idea of supplying Turkey and Greece with new or upgraded US-made fighter aircraft as an enticement for Turkey to let Sweden join NATO.

"And so, what I'm trying to, quite frankly, put together is a little bit of a consortium here where we're strengthening NATO in terms of the military capacity of both Greece as well as Turkey and allow Sweden to come in," Biden said.

"But it's in play. It's not done."

While in Vilnius, Biden will also deliver a major foreign policy speech at the city's university.

His trip comes in the wake of a controversial decision to supply Ukraine with cluster munitions, which most NATO member countries have banned but which the United States continues to use and says will help Ukraine destroy heavily dug-in Russian forces.

Biden's final stop before returning to Washington on Thursday will be Finland, which ended its historic neutrality to enter NATO in response to the Russian attack on Ukraine.

Biden will be the first US president to visit Helsinki since Donald Trump went five years ago to meet with Russian President Vladimir Putin.