Mr Biden told Prime Minister Rishi Sunak he "couldn't be meeting with a closer friend and greater ally" as the pair met this morning in Downing Street.

He joined the prime minister in the Downing Street garden at the start of their talks, after greeting him at the door to Number 10.

The prime minister said it was "great for us to carry on our conversation" following a string of meetings this year, including last month's visit to Washington.

Mr Sunak said the pair would consider "how we strengthen our co-operation, joint economic security to the benefit of our citizens".

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Both leaders will also attend this week's NATO summit in Lithuania.

Mr Sunak said: "We stand as two of the firmest allies in that alliance and I know we'll want to do everything we can to strengthen Euro-Atlantic security."

It comes amid reports of disagreements regarding Ukraine's ambitions for NATO membership and the US decision to provide Kyiv with cluster munitions, which the UK does not support.

But Mr Biden insisted: "Our relationship is rock solid."

Shortly after meeting with Mr Sunak in Downing Street, Mr Biden headed to Windsor Castle in Berkshire for tea and talks with the King.

It was also confirmed there would be a "high-level" meeting between Number 10 and the White House in October to "drive progress" on the Atlantic Declaration - an agreement to bolster economic security in response to the growing threat of China.

Earlier, Downing Street played down reports of a UK-US split, saying Ukraine's desire to join NATO would be discussed at the summit in Lithuania later this week.

While the UK supports Ukraine's desire for a speedy accession to the military alliance, the US has struck a more cautious tone, with Mr Biden saying on Saturday that Ukraine was "not ready" for NATO membership currently.

Asked about the divergence of view, the Number 10 spokesman said: "Certainly, we do want to support Ukraine on the pathway to joining the alliance, the exact mechanisms for that are for discussion with Nato allies."

He added that he had seen reports of a disagreement, but added: "I don't believe that's accurate."

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Another issue that has divided Downing Street and the White House is the latter's decision to supply cluster munitions to Ukraine.

The UK is signed up to an international convention to ban the weapons, placing a duty on Mr Sunak to speak out against their use. The US is not signed to the agreement.

The spokesman said providing the weapons "was a difficult choice for the US" that had been "forced on them by Russia's war of aggression".

The two leaders "discussed the commitments that UK has under that convention, both not to produce or use cluster munitions and to discourage their use".

Asked if Mr Sunak complied with that commitment to discourage the use of the weapons in his talks with Mr Biden, the spokesman said: "Yes, they discussed the requirements the prime minister is under because of this convention, and the UK is upholding that."