Suspected Sunni insurgents bombed and badly damaged a span over the main north-south highway leading from Baghdad on Tuesday - 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 Shi'ite-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 the blast. The explosion at 7:30 a.m. Tuesday - not thought to be a suicide bomb - struck a bridge linking the villages of Qariya al-Asriyah and Rashayed in northern Babil province, 60 kilometers south of Baghdad. No injuries were reported. About 60 percent of the bridge was damaged, but one lane was passable, police said. But debris from the blast fell on the main north-south expressway below, further complicating efforts to reopen that main artery, closed after Sunday's blast dropped masses of concrete onto the roadway. On Monday, a parked truck bomb destroyed a bridge carrying traffic over the Diyala River in Baqouba, 60 kilometers (35 miles) northeast of Baghdad. No casualties were reported, but vehicles were being forced to detour to a road running through al-Qaida-controlled territory to reach important nearby cities. Violence persisted elsewhere on Tuesday, much of it in Baqouba, the capital of Diyala province, which is swarming with al-Qaida fighters. The al-Qaida militants were driven out of Baghdad by the four-month-old US security operation, and out of Anbar province - west of the capital - by Sunni tribesman who rose up against the terrorist group. 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, speaking on condition of anonymity because of security concerns. The fighting also prevented university students at nearby colleges from taking final exams, according to the provincial police center. Passengers on minibuses in Baqouba and surrounding areas also came under fire in at least three drive-by shootings that left six civilians dead and two wounded, the police officials said. Elsewhere in the province, gunmen stormed the house of the Sunni mayor of Muqdadiyah, about 90 kilometers (60 miles) north of Baghdad, forcing the family members outside, then blowing up the house, according to the police officials. Najim al-Harbi, a member of the Iraq Islamic Party, was not home at the time. In other violence reported by police, a Sunni Arab medical student was killed in a drive-by shooting in the northern city of Mosul, and a roadside bomb killed two people and wounded two others in central Baghdad. The attacks on the bridges were only the latest in a bid to deepen turmoil in Iraq, especially on the vital transportation network linking Baghdad to the rest of the country. Such bombings - especially suicide attacks - are an al-Qaida trademark and one of the group's many and ever-shifting tactics against US and Iraqi forces. Earlier this month, a bomb heavily damaged the Sarhat Bridge, a key crossing 145 kilometers (90 miles) north of Baghdad on a major road connecting the capital with Irbil, Sulaimaniya and other Kurdish cities. In March and April, three of Baghdad's 13 bridges over the Tigris River were bombed. The attacks were blamed on Sunni insurgent or al-Qaida attempts to divide the city's predominantly Shi'ite east bank from the mostly Sunni western side of the river. The most serious attack, an April 12 suicide truck bombing, collapsed the landmark Sarafiyah Bridge and sent cars plunging into the brown waters of the Tigris. Eleven people were killed. Paul Kane, a fellow with the International Security Program at Harvard University's Kennedy School of Government, said the attacks on bridges were an extension of earlier insurgent attacks on "electric generation sites, infrastructure for water and also the obvious target of oil pipelines." Kane noted that Iraq did not have railroad service, so insurgents "may be at the end of the transit list. If anything, it means they're trying to be creative and they're running out of targets." Al-Maliki told Negroponte that the Shi'ite-led government was determined to gain passage of draft laws on equitably distributing the country's oil wealth and allowing thousands of former members of Saddam Hussein's ousted Baath Party to return to government jobs and join the military, according to a statement from his office. The Iraqi government and parliament were under heavy pressure from the Americans to pass the legislation, which is seen as a way to bring the disaffected Sunni minority into the political process in a bid to stem support for the insurgency. The US Embassy confirmed that Negroponte had met with senior US and Iraqi officials in Baghdad but provided no details. The New York Times reported Tuesday that Adm. William Fallon, the top US commander in the Middle East, warned al-Maliki on Sunday that the Iraqi government needed to make tangible political progress by next month to counter the growing tide of congressional opposition to the war. The al-Qaida front group Islamic State in Iraq, meanwhile, posted a video showing what it said were 14 captive members of the Iraqi security forces and threatened to kill them in 72 hours if their demands were not met, including the release of all female prisoners in Iraqi prisons. The hostages were shown in uniform standing in three rows; one of them repeatedly sighed and looked up at the ceiling. It wasn't clear when they were seized.