Rishi Sunak has said the UK “discourages” the use of cluster bombs after the US agreed to send them to Ukraine.
President Joe Biden has faced criticism for supplying the munitions, which are banned by many allies because of their track record of killing and maiming civilians.
The prime minister said the UK was one of 123 countries that signed a convention banning their use, and would continue focusing on supplying tanks and long-range weapons to help the fight against Russia.
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He added: “We will continue to do our part to support Ukraine against Russia’s illegal and unprovoked invasion, but we’ve done that by providing heavy battle tanks and most recently long-range weapons, and hopefully all countries can continue to support Ukraine.
“Russia’s act of barbarism is causing untold suffering to millions of people.
“It’s right that we collectively stand up to it.”
Mr Sunak will meet Mr Biden in London on Monday ahead of a NATO summit.
Why cluster bombs decision is controversial
Cluster bombs detonate in the air and release “bomblets” that scatter over a large area.
Opponents say they kill indiscriminately and that some of the smaller munitions can fail to detonate, posing a long-term risk to civilians.
Mr Biden has called it a “difficult decision” but said he had to act as “the Ukrainians are running out of ammunition”.
The US says Kyiv has provided assurances it will not use cluster bombs in urban areas, but some NATO allies are likely to be uneasy over their supply.
Why US cluster bombs decision is already causing splits among allies
The US decision to send cluster munitions to Ukraine is extremely controversial and is already causing splits in the alliance.
The problem is many of the allies – including the UK – are signatories to a convention banning their use.
And the reason for that is simple – as weapons they are absolutely horrific, hanging around long after the fighting has ceased with the potential to kill indiscriminately.
The US has got Kyiv to agree not to use the munitions in built-up areas but that’s hardly the point – and it’s why there is great unease about their supply.
Cluster munitions describe a method of dispersing large numbers of bomblets over a wide area – they can be delivered by missile, rocket or artillery shell.
They are meant to explode on impact but many don’t, meaning they remain where they fall and can be trodden on by civilians.
They are thought extremely effective as an area weapon, especially when used against heavily fortified positions.
And with Ukraine’s counteroffensive making slow progress the calculation in Washington seems to be that they could help advancing forces to break Russian lines.
Ukraine is also running low on artillery shells.
Neither Russia nor Ukraine are signatories to the treaty and both have used them since the start of the invasion.
The political dimension to all this is awkward but it’s unlikely to derail the alliance.
Downing Street has made its position clear and that will probably be the end of the matter.
There’s though, a real danger of the moral ground the West considers itself to stand on, on the issue of Ukraine, becoming eroded.
Russian war crimes are well documented but Washington is certainly opening itself up to accusations of hypocrisy and double standards.
Some human rights groups consider even the use of cluster munitions as a war crime.
The Convention on Cluster Munitions prohibits their use or stockpiling because of their indiscriminate effect on civilian populations, though the US, Ukraine, and Russia are not signatories.
Both Moscow and Kyiv have used cluster munitions so far in the war.
Ukrainian President Volodymyr Zelenskyy thanked the US for the “timely, broad and much-needed defence aid package” that will “bring Ukraine closer to victory over the enemy, and democracy to victory over dictatorship”.
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Opponents say cluster bombs kill indiscriminately and pose a long-term risk to civilians. File pic
Biden’s made ‘wrong call’
Commons Defence Committee chairman Tobias Ellwood urged the US to “reconsider”.
The Tory MP tweeted: “This is the wrong call and will alienate international good will. Their use leaves deadly unexploded ordnance over the battlefield, killing & injuring civilians .”
But Dr Yuriy Sak, adviser to Ukraine’s defence minister, told Sky News: “It is an important move. It is a timely move.
“Russia has been using cluster munitions from day one of this aggressive, unprovoked war against Ukraine. They’ve been using cluster munitions against civilian cities.
“Now, we need these types of weapons as well as other types of weapons, in order for us to achieve one single sole purpose, which is to liberate our land.”
He added: “We will be using these weapons very responsibly.
“Our minister of defence has given written assurances that certain principles will be applied.
“We have a right to self-defence… but in exercising that right, we will always commit ourselves to abiding the international norms of war and humanitarian laws.
“We are dealing with an enemy who is killing our civilians every day.”
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‘The only target will be the enemy’
Insisting the weapons would not be used in urban areas, Dr Sak said: “They will be used only on the battlefield to allow the Ukrainian army to break through the defence lines of the enemy.
“The only target will be the enemy.”
Germany, which is signed up to the cluster bomb ban, has expressed understanding for the American position.
A spokesman for the Berlin government said: “We’re certain that our US friends didn’t take the decision about supplying such ammunition lightly.
“We need to remember once again that Russia has already used cluster ammunition at a large scale in its illegal war of aggression against Ukraine.”
But Spain, also a signatory to the convention, said it opposed the decision.
Spanish defence minister Margarita Robles said: “Spain, based on the firm commitment it has with Ukraine,
also has a firm commitment that certain weapons and bombs cannot be delivered under any circumstances.”
