US forces launched a big offensive in Iraq with an overnight airborne assault targeting Al Qaeda guerrillas on Tuesday, part of a major new countrywide push.

The Americans also raided Baghdad's Shi'ite slum of Sadr City targeting militants they said are linked to Iran. Relatives said a 5-year-old girl was among four killed in the raid.

A suicide truck bomb killed 10 people and destroyed an important bridge linking Baghdad to the north, and 15 corpses identified as Sunni Arabs were found dumped by a highway.

The military said 16,000 US and Iraqi troops were involved in Operation Lightning Hammer against Sunni Islamist al Qaeda in the fertile crescent of the Diyala River north of Baghdad.

US and Iraqi soldiers started the operation with a late-night air assault, focusing on militants who fled an earlier crackdown in the provincial capital Baquba.

"Our main goal with Lightning Hammer is to eliminate the terrorist organisations ... and show them that they truly have no safe haven -- especially in Diyala," Major-General Benjamin Mixon, US commander in northern Iraq, said in a statement.

The operation was described as part of a larger countrywide Operation Phantom Strike, which US forces announced on Monday.

Al Qaeda is seen trying to influence debate in Washington by stepping up attacks in Iraq before a crucial progress report on the war is delivered to Congress on Sept. 15.

Pentagon spokesman Bryan Whitman said US forces would launch a series of operations over the next 30 days.

"We fully expect that al Qaeda in Iraq would like to increase their attacks during this critical period," he told reporters in Washington.

US military offensives have been under way in Baghdad in recent months and surrounding provinces like Diyala, a sectarian patchwork, has seen some of Iraq's worst violence.

Police in the Diyala town of Khalis said they found 15 corpses identified as Sunni Arabs, executed by gun shots and dumped on the highway linking Baghdad and Kirkuk.

A bridge over a branch of the Tigris River collapsed when a truck bomb exploded at the middle, sending three cars into the water, police said. Ten people were killed and six wounded.

Police and an oil industry source said gunmen had also kidnapped a senior official of Iraq's state oil marketing organisation, although details of the incident were sketchy.

The United States has sent an additional 30,000 troops to Iraq this year and moved them from large bases into small neighbourhood outposts in an effort to reduce sectarian violence in the capital and surrounding provinces.

US forces say they have had success, especially against Sunni Arab militants who were their main enemies for the first three years after the fall of President Saddam Hussein in 2003. But they have faced more violence from Shi'ite militia, who they say have ties to neighbouring Shi'ite Muslim Iran.  -Reuters