By Steve Holland

DOVER AIR FORCE BASE, Delaware U.S. President Joe Biden departed on Sunday on a three-nation trip that will be dominated by a NATO summit in Lithuania aimed at showing solidarity with Ukraine in its fight against Russia while not yet accepting Kyiv as a member of the alliance.

Biden's first stop will be in London, where he will meet British Prime Minister Rishi Sunak at 10 Downing Street on Monday and then travel to Windsor Castle for a visit with King Charles.

The talks with the king, expected to include climate initiatives, will give Biden a greater sense of the man who succeeded his mother, Queen Elizabeth, who died last September.

Biden had tea with the queen at Windsor in June 2021 and they discussed many of the same issues that remain a top priority today, like Russia and China.

Biden will travel on to Vilnius, Lithuania, on Monday night and hold talks with NATO leaders there on Tuesday and Wednesday. Biden and the allies aim to show support for Ukraine and give Ukraine President Volodymyr Zelenskiy a sense of what he will have to do to gain NATO membership at some point in the future.

In a CNN interview previewing his trip, Biden urged caution for now on Ukraine's drive to join NATO, saying the alliance could get drawn into the war with Russia due to NATO's mutual defense pact.

"I don't think there is unanimity in NATO about whether or not to bring Ukraine into the NATO family now, at this moment, in the middle of a war," Biden said.

Zelenskiy said an invitation for Ukraine to join NATO would send a message that the Western defense alliance is not afraid of Moscow. Ukraine should get clear security guarantees while it is not in NATO and that would be one of his goals in Vilnius, he added in an interview broadcast Sunday.

"I'll be there and I'll be doing whatever I can in order to, so to speak, expedite that solution, to have an agreement with our partners," Zelenskiy said on ABC's "This Week."

A centerpiece of Biden's visit to Lithuania will be a speech that he will deliver at Vilnius University on Wednesday night.

White House national security adviser Jake Sullivan told reporters the speech will cover Biden's vision of "a strong, confident America flanked by strong, confident allies and partners taking on the significant challenges of our time, from Russia’s aggression in Ukraine to the climate crisis."

One of Biden's objectives on the trip is to show Americans back home the importance of continuing support for Ukraine as he faces re-election. Some of his Republican rivals in the race for the November 2024 presidential elections have voiced doubts about his strategy.

Solid majorities of Americans support providing weaponry to Ukraine to defend itself against Russia and believe that such aid demonstrates to China and other U.S. rivals a will to protect U.S. interests and allies, according to a Reuters/Ipsos survey late last month.

Biden's last stop will be in Helsinki for talks with the leaders of the newest NATO member, Finland, and to attend a summit of U.S. and Nordic leaders.

(Reporting by Steve Holland; Editing by Franklin Paul and Andrea Ricci)