US President Joe Biden dropped in for tea and climate change talks with King Charles III on Monday, after a garden meeting with UK Prime Minister Rishi Sunak before a NATO leaders' summit on Ukraine.

Biden, 80, flew in his Marine One helicopter from Downing Street in central London to Windsor Castle, touching down for a ceremonial welcome from a band of red-jacketed Welsh Guards.

"The Star Spangled Banner" and "God Save the King" rang out across the manicured lawns of the stately royal residence west of London, before the pair disappeared inside.

It was Biden's first meeting with Charles since he was officially crowned king in May. The US president did not attend the coronation but was represented by his wife, First Lady Jill Biden.

The couple had been at the state funeral of Charles's mother, Queen Elizabeth II, in September last year, while Biden visited the late queen at Windsor after the G7 summit in 2021.

Charles, 74, is a lifelong environmentalist and Biden considers him a "clarion voice" on climate change, White House national security adviser Jake Sullivan told reporters on Sunday.

Top financiers and philanthropists are also meeting in Windsor to discuss climate finance to boost support for developing nations to cut carbon emissions.

Attending the meeting are UK energy security and net zero minister Grant Shapps and Biden's special climate envoy John Kerry.

The UK government said more than $1 trillion is needed by 2030 to help developing economies get on track to reach net-zero carbon emissions by 2050.

Additional investment, including from the private sector, is required to cut non-CO2 emissions, halt deforestation as well as to adapt and build resilience to climate change.

Biden, who arrived in the UK on Air Force One late Sunday, is heading to the Lithuanian capital, Vilnius, for the two-day NATO meeting.

- Ukraine talks - 

Biden's flying visit to the UK came with Ukraine topping the agenda at the upcoming alliance meeting, as Kyiv pushes to join once its war with Russia is over.

Western allies agree that Ukraine cannot join while war is still raging, as it would pull NATO into direct conflict with nuclear-armed Russia.

But the United States is seen as most hesitant about its membership. Biden has described Kyiv's bid as "premature". 

"I don't think it's ready for membership in NATO," he told broadcaster CNN.

Nevertheless, a Western official told AFP the allies were "set" to drop the Membership Action Plane requirement for Ukraine's application.

Biden and Sunak held talks for just under 40 minutes, seated in wicker chairs and sipping from specially branded Downing Street mugs.

Downing Street said the pair "agreed on the need to strengthen our alliance and maintain our support for Ukraine".

Sunak also played down any talk of a rift after some Western allies questioned the US supply of cluster munitions to Ukraine, and concern from rights groups about their dangers to civilians.

The prime minister's official spokesman told reporters afterwards the issue was discussed, acknowledging that providing the weapons "was a difficult choice for the US".

But it had been "forced on them by Russia's war of aggression", he added.

Biden and Sunak have met regularly in recent months, as the UK leader tries to repair transatlantic ties strained by the turbulent tenures of Boris Johnson and Liz Truss.

Irish-American Biden's position on UK wrangling with the European Union over post-Brexit trade in Northern Ireland has been seen as at the heart of the tensions.

Relations have improved since Sunak and Brussels struck a deal about the movement of goods, protecting a hard-won peace after 30 years violence over British rule in the province.

Biden told reporters he "couldn't be meeting with a closer friend and greater ally", adding: "Our relationship is rock solid."

From Lithuania, Biden travels to Finland, NATO's newest member, for a US-Nordic Leaders' Summit.

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