The deaths were not a result of enemy reaction, the U.S. command said, but they raised 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, 80 kilometers (50 miles) west of Baghdad in Anbar province where many of Iraq's Sunni Arab-led insurgent groups are based.

"We are a close-knit family and this loss affects us all," said U.S. 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 immediately 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 a U.S. soldier died Tuesday from non-combat related wounds.

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. The former is responsible for the Iraqi military, the later Iraq's police forces.

U.S. officials hope a new unity government can win public confidence and quell the violence so that American and other international troops can go home.

But 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 Arab 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, 25 miles (40 kilometers) northeast of Baghdad, when dozens of gunmen, some of them wearing military uniforms, raided and abducted 10 young men.

Village leaders and clerics alerted police and U.S. soldiers, who rushed to the scene, clashed with the gunmen and rescued seven of the hostages, police said. Three others were missing and presumed taken away by gunmen, police said.

U.S. troops killed at least one kidnapper and wounded another, said Lt. Col. Thomas Fisher, commander of the 1st Battalion, 68th Armor. Some of the hostages had been severely beaten, he told Associated Press TV News.

More than 30 people were taken into custody, Iraqi police said, and interrogators were trying to determine their identities. Some gunmen told police they belong to the militia loyal to radical Shiite cleric Muqtada al-Sadr and had come from Baghdad, Iraqi authorities said.

Kidnappings are believed to have risen steadily since the U.S.-led invasion of 2003, although police believe few are reported. A study by the Brookings Institution estimated that between 30 and 40 Iraqis were kidnapped per day in the Baghdad area alone during March, compared with two a day in the capital in January 2004.

Fisher said the incident may have been "tribal in nature." He did not elaborate, but tensions have been running high for months between Shiite and Sunni communities in religiously mixed Diyala province.

With the rise in sectarian tensions, much of the violence has shifted from Sunni insurgent strongholds such as Anbar province to Baghdad and other areas with a mixed population.

The shift has impacted heavily on civilians, many of whom have been targeted simply because of their religious affiliation. According to the Health Ministry, 952 people were killed nationwide last month in "terrorist" violence, among them 686 civilians.

By comparison, ministry figures showed that 548 civilians were killed nationwide in January, 545 in February and 769 in March.