LONDON: U.S. President Joe Biden will look to deepen his relationships with King Charles and British Prime Minster Rishi Sunak during separate meetings on Monday in which Ukraine and climate change are expected to dominate the agenda.

Biden landed in London on Sunday night to kick off a three-nation trip that will feature 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 defense alliance.

On Monday at Windsor Castle, the 80-year-old president and the 74-year-old king will discuss how to help boost private investment to combat climate change, a threat both leaders say is existential.

"The president has huge respect for the king's commitment on the climate issue in particular. He has been a clarion voice on this issue," White House national security adviser Jake Sullivan told reporters aboard Air Force One on Sunday.

Sullivan said Biden hopes to deepen his personal relationship with King Charles. He said the two leaders - who don't know each other well - had a phone conversation earlier this year that he described as "incredibly warm."

Biden will also travel to 10 Downing Street on Monday to hold a low-key meeting with Sunak, their fifth in as many months. Sullivan said the visit is more of a continuation of a long-running conversation than formal meeting. It will mark Biden's first visit to Downing Street as president.

Sullivan said the two leaders will share notes ahead of the NATO summit in Lithuania, which kicks off on Tuesday and will be dominated by the Ukraine crisis, which has brought the alliance closer.

Ahead of the 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 in a CNN interview that aired Sunday.

Biden's trip comes days after he agreed to send controversial U.S. cluster munitions to Ukraine.

Cluster munitions are prohibited by more than 100 countries, including the United Kingdom, due to their threat to civilian populations because they typically release large numbers of smaller bomblets that can kill indiscriminately over a wide area.

Russia, Ukraine and the United States have not signed on to the Convention on Cluster Munitions, which bans production, stockpiling, use and transfer of the weapons.

"I think you find Prime Minister Sunak and President Biden on the same page strategically on Ukraine, in lock step on the bigger picture on what we are trying to accomplish, and as united as ever," Sullivan said on Sunday.