US President Joe Biden has landed in the UK ahead of a Nato summit in Lithuania later this week - which comes after several allies questioned his call to send cluster bombs to Ukraine.
The UK and Canada are among those who voiced concern about supplying the bombs, which are widely banned because of the danger they pose to civilians.
The US says they are needed because Ukraine's weapon stocks are dwindling.
On Monday, Mr Biden will meet UK Prime Minister Rishi Sunak.
The two men are expected to discuss various issues, including the war in Ukraine.
Mr Sunak has not directly criticised his US counterpart following Friday's cluster bomb announcement - but on Saturday he said that the UK was one of 123 countries signed up to the Convention on Cluster Munitions, an international treaty which bans the production or use of the weapons.
Other US allies have gone further, however. Nato partner nation New Zealand said on Sunday the munitions could cause "huge damage to innocent people".
Cluster bombs typically release lots of smaller bomblets that can kill indiscriminately over a wide area. Unexploded bomblets can linger on the ground for years before they detonate.
The US says it has received written reassurances from Kyiv that Ukrainian troops will not use the weapons in Russia or in urban areas.
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