The weapon detonates in the air and releases "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.

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US officials said the cluster bombs it will provide have a low "dud rate" of under 2.35% to minimise the risk.

Mr Biden called it a "difficult decision" but said he had to act as "the Ukrainians are running out of ammunition".

He told CNN the cluster bombs were being sent as a "transition period" until it can supply more standard 155mm artillery.

Colin Kahl, under-secretary of defence for policy, said "hundreds of thousands" were available but refused to state how many would initially be provided.

He said Russia had been using older cluster munitions with a dud rate of 30-40% since the start of the war.

Ukraine is said to have given written assurances not to use them in populated areas, to map where they are used, and committed to a post-war clean-up.

The US put off the decision "as long as we could" due to the risk to civilians, said Jake Sullivan, Mr Biden's national security adviser.

But he said there was "a massive risk of civilian harm" if Russia takes more territory because Ukraine doesn't have enough artillery.

Ukrainian presidential adviser Mykhailo Podolyak said cluster munitions could have an "extraordinary psycho-emotional impact" on demoralised Russian troops.

More than 100 countries are signed up to a convention against the use and manufacture of cluster bombs, but the US, Russia, and Ukraine are not part of it.

"We're not signatories to that agreement, but it took me a while to be convinced to do it," Mr Biden told CNN.

He said he'd followed the recommendation of US defence officials.

NATO Secretary-General Jens Stoltenberg said the alliance had no position on the issue and that it was for "individual allies… to make those decisions".

However, the United Nations has urged both sides not to use them.

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UN human rights office spokesperson Marta Hurtado said "the use of such munitions should stop immediately and not be used in any place".

The International Red Cross said cluster munitions "cause significant numbers of preventable civilian casualties".

Oleksandra Ustinova, a member of Ukraine's parliament, is one of those supporting the weapon's use.

She said troops already had to disable mines when they regain territory, and could catch any unexploded bomblets as part of that.

"We will have to de-mine anyway, but it's better to have this capability," she said.

Sky defence analyst Sean Bell said Ukraine had to make a judgement about "the benefits of these weapons, which could provide a decisive capability in this war, versus the risk of the legacy they're going to inherit of having to clear these up".

The last large-scale US use of the weapon was in Iraq in 2003.

However, Human Rights Watch estimates the American-led coalition also used 1,500 cluster bombs in the first three years of the Afghanistan war.

The cluster bombs being sent to Ukraine are part of another tranche of weaponry and ammunition that the US says brings its total arms contribution to $41.3bn (£32bn).