The children, aged five and two, are too stunned to talk about it, and their father and mother were inside at the time.

In a poor city, they live in the poorest quarter of all, where the closest things to toys are bits of plastic scavenged from the rubbish that covers the muddy ground.

“They thought it was a kind of ball,” said Hala’s aunt, weeping. “They only wanted to play.”

But the object picked up by the two children was a bomblet from an unexploded cluster bomb. It went off in the front yard, leaving a neat six-inch hole in the concrete floor.

Tiny fragments of shrapnel flew upwards into Hala’s legs and into Ali’s face. At least one of them is still lodged deep in his cheek. Their father clutches the screaming boy, weeping silently.

Callous though it sounds, they are lucky to be alive. Just the day before, three boys, aged between 7 and 14, were killed, and two injured in a similar tragedy just 500 yards away.

After a quarter of a century of dictatorship, 12 years of sanctions and one of the bloodiest battles of the three-week war, the people of liberated al-Nasiriyah face a new source of misery: unexploded American cluster bombs.

Al-Tadhiya slum is in the centre of al-Nasiriyah, but for the past month it has literally been a minefield.

Yesterday morning, within half a mile of the funeral tent where people were paying their respects to the families of the dead boys, at least eight cluster bombs, along with two unexploded mortar rounds, were visible.

Three were half-buried in the mud, three lay in rubbish next to a house and two were on a nearby roof. Each one is capable of killing, blinding and severing legs and arms. And these are only the ones which have been spotted.

Geoff Hoon, the Defence Secretary, said in the House of Commons during the war that in certain situations cluster bombs are of great military use. Against moving targets, such as armoured columns, their scatter-gun effect is far more useful than conventional artillery. But in urban settings like this one the suffering they cause is incalculable.

A high proportion of the bomblets — some say one in ten, some a quarter — do not go off and lie where they fall, capable of exploding at any moment. At least one US Marine has been injured after tripping over an unexploded cluster bomb. Iraqis hate them. The US troops do not like them. They kill and maim children. So why have they been used?

The local people say that there were indeed Iraqi army units and Fedayin militia in this area, firing on the Americans. But why were cluster bombs, the least precise of all munitions, used against them, rather than aimed mortars and missiles? A nearby group of Marines on patrol suggest an answer.

Lance Corporal Matthew Gamel and Sergeant Jason Daniels have fought their way from Umm Qasr, past Basra and into al-Nasiriyah, and they do not welcome the continuing peacetime danger that the cluster bombs present. They explained the principle behind the weapons — the canister fired from a howitzer, that opens in the air releasing the bomblets; the “shaped charge” that punches a hole through armour; and the shrapnel that sprays anyone within range.

“They are a huge pain in the ass,” the sergeant said. “The only way to get rid of them is to explode them one by one. What I heard is that they began using the cluster bombs because they ran out of high- explosives.” It is easy to imagine: flustered artillery officers, under fire themselves, and under pressure to keep up the barrage which protects the lives of their own troops.

A sudden panic as the conventional shells run out — a hurried decision to load the cluster bombs. And now the harvest — the dozens of explosive toys, each lying in wait for its own Hala and Ali.

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