Joe Biden was given a warm welcome on the steps of Downing Street by Rishi Sunak - amid a huge international backlash over the use of cluster munitions in Ukraine.
The two leaders smiled and shook hands before heading to the garden to crunch talks ahead of a Nato summit in Lithuania tomorrow.
Mr Biden is making a fleeting visit to the UK which will see him meet the King in Windsor this afternoon.
Ahead of the half-hour meeting Mr Sunak was urged to press Mr Biden on the decision to send cluster munitions to Ukraine - having said he "discourages" their use.
US President told the PM he "couldn't be meeting with a closer friend and greater ally". He added: "Our relationship is rock solid."
They are also expected to discuss Ukraine's bid to join NATO, which the US opposes as it is currently at war.
No10 has refused to say whether the PM will raise the issue of cluster bombs during their talks.
The US has come in for heavy criticism after announcing it would be sending the weapons - which dozens of nations have outlawed because of the way they kill and maim civilians.
Mr Sunak responded by saying Britain "discourages" their use as one of 123 signatories of a convention banning the bombs.
Daniel Balson, Amnesty International USA’s advocacy director for Europe and Central Asia said: “Cluster munitions are an indiscriminate weapon that presents a grave threat to civilian lives, even long after a conflict has ended.
"Their transfer and use by any country under any circumstances is incompatible with international law.”
The White House said the pair will "compare notes" on the Ukrainian counter-offensive ahead of a NATO summit in Vilnius tomorrow.
The US opposes Ukraine joining during the war, a move which would pull the wider West into direct conflict with Russia, while the UK has said it supports a fast-track approach.
Mr Biden has described Kyiv's bid as "premature", telling CNN: "I don't think it's ready for membership in NATO."
Meeting Mr Sunak, the president is likely to raise his concerns over post-Brexit challenges to the peace process in Northern Ireland during their fifth meeting in as many months, a US official said.
Mr 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 points of discussion.
Though it is not a full-blown state visit, Mr Biden will then 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 meet with attendees of a climate finance mobilisation forum, where finance and philanthropic leaders will have discussed the support they can offer to poorer nations.
Mr Biden and Mr Sunak will both be at the Nato talks centring on Ukraine and its bid to join the defence alliance.
The president is also likely to face questions from allies on why he is sending cluster munitions to Kyiv when two thirds of Nato members have signed the Convention on Cluster Munitions.
It prohibits the use, stockpiling or transfer of the munitions, which deploy a large number of bomblets across a wide area, because of the enduring danger they pose to civilians.
Defending the move, Mr Biden sought to justify the bombs as needed to fortify Ukraine's depleting ammunition stocks after receiving assurances about their use from Kyiv.
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