US President Joe Biden has arrived in Downing Street for a meeting with Prime Minister Rishi Sunak.
This could include discussions on cluster munitions and Ukraine’s bid to join Nato.
The American leader will then head to Windsor Castle for tea and talks on the climate crisis with the King.
There was a heavy security presence in Whitehall ahead of Biden’s arrival.
The talks kick off a busy day for the president, who is making a layover in London on the way to the Nato summit in Vilnius, Lithuania.
It comes after Biden defended the “difficult” decision to send cluster munitions to Kyiv.
Sunak responded by saying Britain “discourages” their use as one of 123 signatories of a convention banning the bombs, but No 10 would not say whether he will raise the issue on Monday.
The White House said the two men will “compare notes” on the Ukrainian counter-offensive as it makes slow but steady progress in driving out the Russian forces.
Biden’s national security adviser, Jake Sullivan, speaking to reporters on Air Force One, said the talks will be a “continuation of a long-running conversation”.
“They will have the chance to compare notes going into the Vilnius summit to talk about the issues on the agenda in Vilnius, to talk about progress in the war in Ukraine, and, of course, to talk about a range of other issues from China to climate to technology to artificial intelligence”, he said.
The Nato summit is likely to see wrangling over Ukraine’s path to membership of the alliance.
Though all allies at the summit in Vilnius on Tuesday agree that Ukraine cannot join during the war, a move which would pull the wider West into direct conflict with Russia, the US is seen as most hesitant over its membership.
Biden has described Kyiv’s bid as “premature”, telling CNN: “I don’t think it’s ready for membership in Nato.”
Britain, on the other hand, has indicated support for a fast-track approach for Ukraine.
At his meeting with Sunak, the fifth in as many months, the President is likely to raise his concerns over post-Brexit challenges to the peace process in Northern Ireland, a US official said.
Biden’s special climate envoy, John Kerry, told the BBC the environment will be a “critical component” of their talks.
Artificial intelligence and trade are also likely topics of discussion.
Biden was making his first visit to No 10 as President.
He met Sunak in the Downing Street garden at the start of their talks.
The PM 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.
Sunak said the pair would consider “how do we strengthen our co-operation and joint economic security to the benefit of our citizens”.
Both leaders will also attend this week’s Nato summit in Lithuania and 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.”
Though it is not a full-blown state visit, Biden will be treated to a display of pageantry at Windsor Castle.
He will receive a royal salute and hear the US national anthem courtesy of the Welsh Guards, before having tea with Charles.
They will also meet attendees of a climate finance mobilisation forum, where finance and philanthropic leaders will have discussed the support they can offer to poorer nations.
