U.S. President Joe Biden met Britain's King Charles on Monday to discuss how to tackle climate change after earlier dropping in to see Prime Minister Rishi Sunak and hail the "rock-solid" friendship with a close ally.

Biden is in Britain at the start of a three-nation trip including the NATO summit in Lithuania, at which allies aim to show solidarity with Ukraine against Russia's invasion while not yet accepting Kyiv as an alliance member.

Both sides played down the president's meeting with Sunak, their fifth in as many months, describing it as a continuation of long-running discussions, though it was Biden's first visit to the prime minister's Downing Street office as president.

"Great for us to carry on our conversations," Sunak told Biden as they sat in the Downing Street garden.

"We’ve got a lot to talk about," Biden replied. “Our relationship is rock-solid. Couldn’t be meeting with a closer friend and a greater ally. We've got a lot to talk about."