Baghdad: Suspected Sunni insurgents bombed and badly damaged a span over the main north-south highway leading from Baghdad yesterday - the third bridge attack in as many days in an apparent campaign against key transportation arteries.

The United States, meanwhile, sent Deputy Secretary of State John Negroponte to Baghdad as pressure increases on Iraq's Shiite-dominated government to carry through on political reforms aimed at bringing the disaffected Sunni minority into the political process and stem support for the insurgency.

"A lot of missions are ahead of us, on top of them is developing our security forces to handle their national roles in fighting the Al Qaida terrorist group, Saddamists and militias to impose law and order in all the country," Al Maliki said during the meeting, which was held in the prime minister's office in Baghdad's fortified Green Zone.

The attack on the bridge occurred six miles south of a bridge brought down on Sunday by what was believed to be a suicide truck bomber. Three US soldiers guarding that bridge were killed in Sunday's blast.

The explosion struck a bridge linking the villages of Qariya Al Asriyah and Rashayed in northern Babil province, 60km south of Baghdad. No injuries were reported.

Violence persisted elsewhere yesterday, much of it in Baquba, the capital of Diyala province.

Clashes broke out between joint US-Iraqi forces and Al Qaida militants in the city throughout the day, leaving three Iraqi soldiers and 15 militants dead, police and hospital officials said.

The fighting also prevented university students at nearby colleges from taking final exams.

Passengers on minibuses in Baquba and surrounding areas also came under fire in at least three drive-by shootings that left six civilians dead and two wounded.