US commanders believe that the female troops may have been specifically targeted.

The co-ordinated attack struck a lorry returning them from duty in the western Iraqi city which was seized from rebels last year.

Two male marines also died in the ambush Thursday night, when a suicide car bomber struck a convoy and gunmen hiding nearby opened fire on survivors.

The attack came as US polls showed sliding support for the continued military presence in Iraq.

President George W Bush hopes to counter this with a televised speech from Fort Bragg military base tomorrow, the first anniversary of the restoration of Iraqi sovereignty. He plans to highlight progress in large parts of Iraq where there is no insurgency, but he will reject recent calls from some Democrat and Republican lawmakers for a timetable for US withdrawal.

The Fallujah ambush has underscored the dilemma facing military chiefs in the deployment of female soldiers.

Military rules bar women from frontline operations, but in Iraq there are no regular frontlines.

Although women are not assigned as members of ground combat units, they fly helicopters, drive lorries, join raids on suspected insurgent hideouts, serve in bomb disposal squads and treat the injured on battlefields - all activities that expose them to lethal risk.

The checkpoint duties that Thursday's victims had been conducting are also crucial because the marines use females to search Iraqi women to avoid upsetting Islamic cultural sensitivities.

This has been particularly important as US forces try to encourage civilians to return to Fallujah after the December offensive against insurgents.

Female troops recently appealed for teddy bears to be donated from the United States so that they can hand them over as a goodwill gesture to Iraqi women and children.

The latest deaths bring to 39 the number of female troops killed since the war began in March 2003.

The first ambush victim named was Lance-Corporal Holly Charette, 21, whose job was to deliver mail.

Eight women were killed in the attack on the Pentagon September 11, 2001, but the biggest previous death toll for servicewomen in a battle zone was in 1945 when six army nurses were killed in a kamikaze attack on their vessel by a Japanese pilot in the Pacific.

About 11,000 women are serving in Iraq alongside 127,000 men.

The Fallujah attack brought the death toll for US military personnel to 1,734 since Iraq was invaded.

Separately, three suicide bomb attacks in the northern city of Mosul killed more than two dozen people Sunday, many of them from the Iraqi security forces, as insurgents kept up pressure on the government, which is backed by the United States.

Within hours a suicide car bomber wrecked a police headquarters, an attack on an Iraqi army base killed up to 16 people and four police were killed when a bomber walked into Mosul's general hospital and blew himself up.

The third attack, on a police post inside the hospital, damaged the emergency ward where casualties had been brought from the previous incidents. Six policemen and nine civilians were wounded.