BAGHDAD, Iraq - The lethal ambush of a convoy carrying female U.S. troops in Fallujah underscored the difficulties of keeping women away from the front lines in a war where such boundaries are far from clear-cut.

The suicide car bomb and ensuing small-arms fire killed at least two Marines and four others were missing and presumed dead. At least one woman was killed and 11 of 13 wounded were female.

The ambush late Thursday also suggested Iraqi insurgents may have regained a foothold in Fallujah, which has been occupied by U.S. and Iraqi forces since they regained control of the restive city from insurgents seven months ago.

The women were part of a team of Marines assigned to various checkpoints around Fallujah. The Marines use females at the checkpoints to search Muslim women "in order to be respectful of Iraqi cultural sensitivities," a military statement said. It is considered insulting for men to search female Muslims.

The terror group al-Qaida in Iraq claimed it carried out the ambush, one of the single deadliest attacks against the Marines - and against women - in this country. The high number of female casualties spoke to the lack of any real front lines in Iraq, where U.S. troops are battling a raging insurgency and American women soldiers have taken part in more close-quarters combat than in any previous military conflict.

"It's hard to stop suicide bombers, and it's hard to stop these people that in many cases are being smuggled into Iraq from outside Iraq," President Bush said at a joint White House news conference with Iraqi Prime Minister Ibrahim al-Jaafari on Friday.

Current Pentagon policy prohibits women from serving in front line combat roles - in the infantry, armor or artillery, for example. But an increasing number of female troops have been exposed to hostile fire.

Thirty-six female troops have died since the war began, including the one that was announced Friday, said Maj. Michael Shavers, a Pentagon spokesman. Thirty-four were Army, one Navy and one Marine. Most have died from hostile fire.

More than 11,000 women are serving in Iraq, part of 138,000 U.S. troops in the country, said Staff Sgt. Don Dees, a U.S. military spokesman.

Thursday's attack may have been the single largest involving female U.S. service members since a Japanese suicide pilot slammed his plane into the USS Comfort near the Philippines in 1945, killing six Army nurses, according to figures from the Women in Military Service for America Memorial Foundation.

Three Army women were among 28 U.S. troops who died during Operation Desert Storm in 1991 when a Scud missile struck a Marine barracks in Dhahran, Saudi Arabia, the Washington-based foundation said on its Web site. The three were among a total of 16 women who died in Desert Storm. Four were killed by hostile fire.

One woman was listed as killed in action during the Vietnam War, two women died in the USS Cole bombing in 2000 and eight military women died at the Pentagon during the Sept. 11, 2001, terrorist attacks, the foundation said.

In Thursday's attack, the Marines were returning to their base, Camp Fallujah, when the ambush took place near the eastern entrance to the city, 40 miles west of Baghdad.

Fallujah is a former insurgent stronghold invaded by U.S. forces at great cost last November. It is also the city where an Iraqi mob hung the mutilated bodies of two U.S. contractors from a bridge.

The State Department says about 90,000 of Fallujah's 300,000 residents have recently returned to the city, which benefited from Saddam Hussein's 23 years in power, as did other cities in the Sunni-dominated area north and west of Baghdad.

Lance Cpl. Holly A. Charette, 21, from Cranston, R.I., died in the attack, the Defense Department said Friday. She was assigned to Headquarters Battalion, 2nd Marine Division, II Marine Expeditionary Force, Camp Lejeune, North Carolina.