The UK is committed to a convention that “prevents” the use of cluster bombs, Rishi Sunak said, after the US agreed to give them to Ukraine.
The prime minister stressed that the UK is one of the countries that has banned the controversial weapons, which have a record of killing civilians.
But he also emphasized that the government will continue to support Ukraine.
US President Joe Biden made what he called a “very difficult decision” to grant them in Kyiv on Friday.
Spain, one of 123 countries that have banned cluster bombs, criticized the decision to send them, which was also condemned by human rights groups.
Cluster munitions are a method of dispersing several small bombs from a rocket, missile or artillery shell that scatters them mid-flight over a wide area.
It is intended to explode on impact, but a significant portion of it may fail to detonate initially – usually if it lands on wet or soft ground. This means they can explode later, killing or injuring people.
Neither the US, Ukraine nor Russia have signed the international treaty – the Convention on Cluster Munitions – that prohibits their use or stockpiling because of the indiscriminate damage they can inflict on civilian populations.
Speaking to reporters in Selby, Yorkshire, on Saturday, Mr Sunak said the UK had “signed a convention banning the manufacture or use of cluster munitions and restricting their use”.
“We will continue to do our part to support Ukraine against Russia’s illegal and unprovoked invasion, but we have done so by providing heavy battle tanks and the latest high-altitude weapons, and hopefully all countries can continue to support Ukraine,” he added.
“The act of barbarism in Russia has caused untold suffering to millions of people.”
Sunak is due to meet with Mr Biden in London on Monday, ahead of the NATO summit in the Lithuanian capital Vilnius on Tuesday.
Rishi Sunak spoke volumes when asked for his response to the US now providing cluster munitions to Ukraine.
He did not criticize the US decision but pointed out that the Convention prohibits the use of bombs.
The UK is the second largest provider of military aid to Ukraine – only behind the US.
As the conflict in Ukraine develops, so do the responses of Kyiv’s allies – on this issue the US and the UK are going in different directions.
Mr Biden justified the supply of weapons by saying that “the Ukrainian people have run out of ammunition”.
Speaking to CNN in an on-air interview on Sunday, he said it “took a long time to convince” to make the “very difficult decision” to send them.
Ukrainian President Volodymyr Zelensky hailed the “deliberate” move to deliver the bombs.
Ukrainian Defense Minister Oleksii Reznikov said his troops would not use them in urban areas to avoid endangering the lives of civilians, adding “these are our people, they are Ukrainians that we have a duty to protect “.
But Spanish Defense Minister Margarita Robles said her government’s position was that cluster bombs should not be used for the “legitimate defense of Ukraine”.
Germany, which is also a signatory to the convention, said that Ukraine would not give it to them but that it understood the American position.
Human Rights Watch said both sides used weapons of war, causing “numerous deaths and serious injuries to civilians”.
These comments were echoed by Amnesty International, which stated that cluster munitions pose “a serious threat to civilian lives, even long after the conflict ends”.
The UN human rights office was also critical, calling for their use to “stop immediately”.
Nato Secretary General Jens Stoltenberg said the military alliance has no position on cluster munitions.
Tobias Ellwood, the chairman of the UK’s defense select committee, urged the US to “reconsider” its decision – which he said was “the wrong call and alienates international goodwill”.
“Their use leaves deadly unexploded ordnance on the battlefield, killing and injuring civilians long after the war ends,” added the Conservative MP.
Russia described the US decision as an “act of desperation” in the face of “the failure of the famous Ukrainian counter-offensive”.