The US President met the UK Prime Minister and King in London ahead of a vital NATO summit.
US President Joe Biden put on a show of unity with London on Monday during a brief visit to the UK.
He met Prime Minister Rishi Sunak and then the King, ahead of a major Nato summit in Lithuania.
Biden was received by Charles III at Windsor Castle for tea and a discussion on the environment.
According to the US Presidency, the two Heads of State were to talk mainly about the climate, a subject on which the King has long been committed and for which the US President has "immense respect".
This will be their first meeting since the coronation of Charles III.
Before heading to Windsor, Biden spent around 40 minutes at Downing Street, where he was greeted on the steps by Rishi Sunak before taking tea in the gardens.
The US president praised the "rock solid" relationship between the two countries. "I couldn't meet a closer friend and a greater ally", he said.
The US and the UK are "two of the firmest allies" within NATO, boasted Rishi Sunak, whose meeting with Biden was his sixth.
The two leaders agreed on the need to "strengthen" NATO and "continue to support Ukraine" so that it can win a "just and lasting peace" in the face of the Russian invasion, according to the Downing Street minutes.
They also shared the hope that Sweden would be able to join NATO quickly and fully.
However, this visit comes two days after the United States controversially decided to supply Ukraine with cluster munitions, which are banned in many NATO countries.
Following some condemnation from European allies, Rishi Sunak pointed out on Saturday that the UK is a signatory to international agreements that ban the production and use of these weapons.
On board Air Force One before Biden's arrival in London, White House National Security Adviser Jake Sullivan reduced the difference in position between Washington and London to a "legal" one.
The UK is "a signatory to the Oslo convention, the US is not", he said.
But Biden and Sunak are "on the same page strategically on Ukraine", he insisted, underlining the good relationship between the pair.
Biden's brief stopover in the UK comes on the eve of a major Nato summit in Vilnius, Lithuania, where Ukraine is hoping to receive new promises of arms deliveries.
According to a Western official, the alliance will remove a major obstacle in Kyiv's accession process.
But on Sunday, the Democrat President dashed Ukraine's hopes of rapid accession. "I don't think it's ready to join NATO", he said in an interview on CNN.
After London and Vilnius, Joe Biden will travel to Finland for a meeting with Nordic leaders.