BAGHDAD, Iraq — Four Marines drowned when their tank rolled off a bridge and plunged into a canal, the military said today, adding that while the accident occurred in a Sunni insurgent stronghold, it was not the result of an enemy action.

The deaths brought to at least 12 the number of U.S. service members who have died in Iraq this week, according to an Associated Press count.

The accident occurred Thursday when the four Marines with Regimental Combat Team 5 were traveling in a U.S. M1A1 Main Battle Tank near Karmah, 50 miles west of Baghdad in Anbar province. U.S.-led coalition forces have seen heavy fighting in the area, known as the Sunni Triangle because it is rife with Sunni insurgents.

"We are a close-knit family and this loss affects us all," said Col. Larry D. Nicholson, commanding officer of Regimental Combat Team 5. "Our thoughts and prayers are with the families of these Marines during this difficult period."

The accident was under investigation, and the military said no other information was available, including what kind of operation the Marines were taking part in and whether fighting with insurgents was under way in the area at that time.

Elsewhere, three U.S. Army soldiers were killed Thursday when roadside bombs hit two U.S. convoys southwest of Baghdad, the military said. The U.S. command also announced that an American soldier died Tuesday from wounds not suffered in combat.

Their deaths raised to at least 2,434 the number of members of the U.S. military who have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.

Three months ago, Iraq's freely elected Parliament took office, but the country's complex mix of Shiite, Sunni-Arab and Kurdish politicians are still trying to form a Cabinet that will make the government fully operational.

The framework was put in place last month with the appointment of Nouri al-Maliki as prime minister-designate. Al-Maliki, a Shiite, is trying to put together the Cabinet, but the process has bogged down over who will lead the defense and interior ministries.

Delays in the political process have led to a surge of sectarian violence, including the kidnapping and killing of civilians by death squads, raising fears of a civil war in Iraq.

That problem was obvious on Thursday, when U.S. and Iraqi forces rescued seven Sunni men seized by suspected Shiite militiamen near Baghdad.

The kidnapping was the latest in a wave that is plaguing the country, killing hundreds of people. Many of the abductions are part of the sectarian warfare in the Iraqi capital, home to large communities of Sunnis, Shiites and Kurds.

Iraqi police said the hostage drama started Thursday morning in two Sunni villages near Khan Bani Saad when dozens of gunmen, some of them wearing military uniforms, raided and abducted 10 young men.