Up to 18 people were killed and many more wounded today when a truck exploded near the house of a police commander in Baghdad, while three US soldiers were shot dead in northern Iraq.

Iraqi police said the cause of the blast was a bomb in the truck, but a US military spokesman said that a tractor-trailer loaded with rockets that were supposed to be fired by Shia militiamen at a nearby US base detonated accidentally.

The blast happened close to the house of police Brigadier-General Nadhim Taeih, in northern Baghdad. His nephew was among between 13 and 18 people killed, but it was unclear whether the commander had been present. More than 60 people were also wounded, including the Brigadier-General's elderly parents.

It was the deadliest explosion in Baghdad in more than two months, bringing down several two-storey houses in the nieghbourhood.

Lieutenant Colonel Steve Stover, the US military spokesman, denied that the carnage had been deliberate.

“They were trying to attack us at that FOB (forwarding operating base), and it went off (accidentally). They wouldn't waste rockets like that,” he said.

Baghdad shook on a near daily basis to the roar of a car or truck bomb in the years that followed the 2003 invasion as a Sunni Arab insurgency took hold, but the sound of explosions is much less frequent now as Iraqi police and soldiers exercise a stronger grip on security.

Nouri al-Maliki, the Prime Minister, has exploited a drop in the violence over the past 12 months to launch a crackdown on Shia militiamen in the southern city of Basra as well as the Baghdad slum of Sadr City.

He is also going after al-Qaeda fighters who have been driven towards the main northern city of Mosul after being pushed out of the capital and the surrounding belts by a surge of US forces.

A decision by thousands of Sunni Arab tribesmen to stop fighting the US military and turn against al-Qaeda also played a big part in this development.

Demonstrating that the situation in Iraq, while improved, remains volatile, three US soldiers were killed in a small-arms fire attack in the town of Hawija, 130 miles north of Baghdad, the U.S. military said. It gave no further details.

The attack, west of the oil-rich city of Kirkuk, is the worst single loss of life for American troops in a month. On May 4, a mine exploded killing four US marines in Anbar province, west of Baghdad. In May, the number of US soldiers killed in Iraq fell to 19, the lowest monthly death toll in the five-year-old war that has so far claimed the lives of 4,090 since the invasion in 2003.

Separately, the US military also said that its soldiers, acting on a tip, yesterday found the decaying bodies of between 10 and 12 people in a sewer shaft in eastern Baghdad. Roughly two years old, the corpses are thought to have been of victims of the sectarian violence that consumed Iraq up until last summer.

US military figures released last month showed that violence in the country is at its lowest level in more than four years.

Iraqi security authorities also reported a sharp decline in the number of Iraqi nationals killed last month, down by almost 50 per cent to 563 compared to April and March.

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