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President Joe Biden met Prime Minister Rishi Sunak on a flying visit to London on Monday, as both readied to join a NATO summit where Ukraine is expected to push to join the alliance once its war with Russia is over.

The 80-year-old US leader stepped out of his presidential limousine "The Beast" and was greeted with a handshake on the steps of Downing Street by Sunak.

The meeting comes just over a month after the pair met in the White House, with the UK eager to maintain that the so-called "special relationship" is as strong as ever.

Biden said the two countries' ties were "rock solid", as he entered Downing Street.

Signs of strained ties have been seen, however, notably in Biden's attitude towards UK wrangling with the European Union over post-Brexit trade in Northern Ireland.

Sunak, struggling to reset the UK government after the turbulent tenures of Boris Johnson and Liz Truss, also wants the UK to be part of future moves to regulate new artificial intelligence technology.

As the summit loomed, there have also been signs of divergence over Washington's provision of cluster munitions to Ukraine, and concern echoed by other Western allies.

Biden said the decision to send the weapons was "very difficult" but Ukrainian forces conducting a counteroffensive against invading Russian troops were "running out of ammunition".

The move raised concerns from rights groups due to the danger unexploded bomblets pose to the civilian population.

Sunak did not directly criticise the United States but reiterated that the UK was one of 120 signatories to an international accord banning the use and supply of cluster munitions.

"We will continue to do our part to support Ukraine against Russia's illegal and unprovoked invasion," he said.

White House national security adviser Jake Sullivan on Sunday played down any rift between the two leaders and in NATO, saying the Biden and Sunak were "on the same page strategically on Ukraine".

Climate talks

NATO leaders are meeting in the Lithuanian capital, Vilnius, on Tuesday and Wednesday, where President Volodymyr Zelensky is hoping for a clear signal that it can join the alliance.

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.

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

Before jetting to Vilnius, Biden will drop in for tea with King Charles III at Windsor Castle, west of London, to discuss climate issues.

It will be 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 funeral of Charles's mother, Queen Elizabeth II, in September last year.

The British monarch, 74, is a lifelong environmentalist and Biden considers that he has been a "clarion voice" on climate change, Sullivan told reporters travelling with the president.

Although not a full state visit, Biden will receive a royal salute at the castle and hear the US national anthem played by a band of the Welsh Guards.

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 will be 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.