London, UK - 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 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.

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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 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 is once again at the top of the agenda as Kyiv pushes to join the alliance once its war with Russia is over.

There's widespread agreement Ukraine cannot join while war is still raging because it would pull NATO into direct conflict with nuclear-armed Russia.

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But the US is considered the 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, and the British PM played down any talk of a rift between allies over the question of the US supplying cluster munitions to Ukraine, amid concern about their dangers to civilians.

Sunak's official spokesperson told reporters 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.