The US military said on Thursday that the offensive dubbed Operation Swarmer was aimed at clearing "a suspected insurgent operating area" northeast of Samarra and was expected to continue over several days.

"More than 1500 Iraqi and Coalition troops, over 200 tactical vehicles, and more than 50 aircraft participated in the operation," the military statement said.

Samarra, about 95km (60 miles) north of Baghdad, was the site of a bombing against a Shia shrine on 22 February that touched off sectarian bloodshed and killed more than 500 and wounded hundreds more.

It is a key city in Salahuddin province, a major part of the so-called Sunni triangle where insurgents have been active since shortly after the US-led invasion three years ago.

Hoshyar Zebari, Iraq's interim foreign minister, said the attack was necessary to prevent insurgents from forming a new stronghold such as they had established in Falluja, west of Baghdad.

"After Falluja and some of the operations carried out successfully in the Euphrates and Syrian border, many of the insurgents moved to areas nearer to Baghdad," Zebari said on CNN. "They have to be pulled out by the roots."

A Shia shrine  in Samarra was
bombed on 22 February

Residents north of Samarra said that there was a heavy US and Iraqi troop presence in the area and that large explosions could be heard in the distance.

They said the operation appeared to be concentrated near four villages - Jillam, Mamlaha, Banat Hassan and Bukaddou - near the highway leading north from Samarra to the city of Adwar. It was not clear whether the operation had met resistance or whether the US aircraft had conducted any attacks.

Waqas al-Juwanya, a spokesman for the provincial government's joint coordination centre in nearby Dowr, said "unknown gunmen exist in this area, killing and kidnapping policemen, soldiers and civilians".

Near the end of the first day of the operation, the military said a number of weapons caches have been captured, containing artillery shells, explosives, bomb-making materials and military uniforms.

Bodies found

Police have meanwhile discovered 29 more bodies discarded in various parts of Baghdad late on Wednesday and Thursday. The victims were all men, some with their hands bound, who had been shot execution-style and dumped in both Shia and Sunni Muslim neighbourhoods, said Lieutenant Colonel Falah al-Mohammedawi.

Iraqis mourn the death of three
schoolgirls killed in Baqouba

North of the capital, a roadside bomb exploded near a girl's primary school near Baqouba, killing three students aged 12-13 and wounding two others, police said.

Another bomb missed a US patrol in Mosul, killing one civilian and wounding three others, police said. Four more people were killed in a drive-by shooting in the city.

In Ramadi, residents picked through the rubble of a home they said was destroyed in a US raid. Residents have reported repeated clashes in the city in an insurgent-plagued area west of Baghdad.

Recent AP Television News video showed a gunbattle in which a gasoline truck was set on fire and at a separate location the killing of an unidentified man with heavy gunfire audible in the background. The US military has not responded to repeated requests for information.