The Marines were killed Thursday while conducting combat operations near the volatile Anbar province Haqlaniyah, 90 miles northwest of Baghdad, the military said in a statement.
The Marines were assigned to the Regimental Combat Team 2, 2nd Marine Division, II Marine Expeditionary Force. Their identities are being withheld pending next-of-kin notification.
As of Friday, at least 1,689 U.S. military members have died since the Iraq war began in March 2003, according to an Associated Press count.
Meanwhile, at least 21 bodies have been found scattered in separate locations near a town close to the Syrian border that is considered an insurgent hotbed, eyewitnesses said Friday.
Eyewitnesses, including an Associated Press reporter and APTN crew, said Friday that one group with 12 bodies had their hands tied behind their backs and were wearing civilian clothes. They were found near a small hamlet called Jabab, about 19 miles east of Qaim. It was unclear when they were killed.
There were another nine bodies found near Qaim outside the village of Fosfat. They were also in civilian clothes and had civilian identification cards.
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It was unclear if the bodies had any relation to a group of about 20 Iraqi soldiers that have been missing from the Qaim area since late Tuesday.
Qaim, an insurgent hotbed 200 miles west of Baghdad, has been the scene of numerous U.S. military and Iraqi army operations. U.S. Marines carried out two major operation in the area last month. A total of 11 Marines were killed in the campaigns.
Al Qaeda in Iraq, the terror group led by Jordanian-born Abu-Musab al-Zarqawi, claimed in an Internet posting that it had abducted a total of 36 Iraqi soldiers in western Iraq on Wednesday. The posting carried on a web site known to carry militant statement could not be independently verified.
"A group of the infidel guards was arrested and investigated Wednesday," It said.
The group added that the men confessed their crimes "against Sunnis and their loyalty to crusaders." To release them, it gave the Shiite-led government of Prime Minister Ibrahim al-Jaafari a day to set free "Muslim women" held in Iraqi prisons. It did not elaborate.
Capt. Ahmed Hamid said the soldiers went missing Tuesday afternoon after leaving an Iraqi army base in two minibuses from Akashat, a remote village near the Syrian border about 70 miles southwest of Qaim.
Hamid, contacted by telephone at an Iraqi military base in Qaim, said the soldiers were wearing civilian clothes and traveling to the Iraqi capital, Baghdad, for a vacation.