A controversial decision by the U.S. to supply Ukraine with cluster munitions has sparked alarm due to the carnage they cause, while Joe Biden admitted it "was not an easy decision."

Human rights groups have condemned the move, citing the long-lasting danger from the weapons which leave behind unexploded munitions and the fact its use in warfare is banned by more than 100 countries. 

Biden said his decision to provide Ukraine with the weaponry was a tough one, but that he believes Kyiv needs them so Russia cannot halt the Ukrainian counteroffensive.

"They're trying to get through those trenches, and stop those tanks from rolling," Biden told CNN. "It was not an easy decision."

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And White House national security adviser Jake Sullivan said "Ukraine has provided written assurances that it is going to use these in a very careful way" to minimize risks to civilians.

What are cluster munitions?

When they detonate, cluster bombs spread dozens of tiny bomblets over an area the size of several football pitches, with a large number burying themselves in the ground rather than exploding. The weapons therefore effectively leave a large field of antipersonnel mines in their wake - prompting a wave of condemnations even before the American delivery was confirmed.

In military terms, cluster bombs allow an army to strike a large enemy formation at a single blow, to deny the use of an airfield's runway or halt an enemy's advance with widespread mines.

Who has banned them?

In total, 123 countries have signed the 2008 Oslo Convention banning production, storage, sale and use of cluster munitions. 

Since their development during World War II, cluster bombs have been used by the U.S. in Iraq and Afghanistan, and by Israel in Lebanon during fighting against Hezbollah in 2006.

"People are still clearing American cluster munitions in Laos" almost 50 years after the war in neighboring Vietnam ended, Chapuis said.

Why are they banned?

The text of the 2008 Convention says cluster bombs "kill or maim civilians, including women and children, obstruct economic and social development... impede post-conflict rehabilitation and reconstruction (and) delay or prevent the return of refugees and internally displaced persons... for many years after use".

 They make no distinction between civilians and military personnel, with experts suggesting between five and 40 percent of bomblets do not explode on impact, remaining present on the battlefield for decades.

Beyond the immediate physical danger posed to civilians, "there is also the question of physical access to affected areas for humanitarian organisations," said HI's Chapuis. The bombs' use can "prevent a lifeline reaching affected populations," he added.

Shipping the weapons to Kyiv would be "escalatory, counterproductive, and only further increase the dangers to civilians caught in combat zones", said Daryl Kimball, director of the US-based Arms Control Association.

"Cluster munitions will not differentiate a Ukrainian soldier from a Russian one," he continued. "The effectiveness of cluster munitions is significantly oversold and the impact on non-combatants is widely acknowledged, but too often overlooked".

The Cluster Munition Monitor, a grouping of several specialist humanitarian outfits, in August 2022 noted that Ukraine was the only theater where the weapons were actively being used - at the time by the Russian army.

Civilians make up 97 percent of people killed or wounded by cluster bombs, with two-thirds of those victims being children where age data is available.

Around the world, 29 countries or territories are known or believed to remain mined with undetonated bomblets, including 10 signatories to the Oslo Convention which are required by the text to de-mine affected areas.

What has been the international response?

Baptiste Chapuis of Handicap International - Humanity and Inclusion said: "This is a death sentence to civilians over the long term. There are people who have not yet been born who will fall victim."

UK Prime Minister Rishi Sunak said it "discourages" the use of cluster bombs.

"The UK is signatory to a convention which prohibits the production or use of cluster munitions and discourages their use," Sunak said in televised remarks. "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."

Spain's defense 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. No to cluster bombs and yes to the legitimate defence of Ukraine, which we understand should not be carried out with cluster bombs."

The U.S. and its allies are looking at ways to provide more aid to Ukraine in its conflict with Russia but why are cluster munitions so controversial?

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