It said the Marines -- three from the 1st Brigade, 1st Armoured Division and the other from Regional Combat Team 5 -- were killed on Thursday. Statements on their deaths gave no further details.

More than 2,570 US soldiers have been killed in Iraq since the March 2003 US-led invasion to topple Saddam Hussein.

Bomb blasts echoed around Baghdad on Saturday as sectarian death squads pursued their bloody work and the US military warned that it was facing stiffer opposition in formerly cooperative Shia areas.

The US troops' most deadly foe remains Sunni insurgents but the coalition is now drawn increasingly into clashes with powerful Shia militias.

This trend is all the more ominous given that US commanders have decided to put around 4,000 additional troops into the mainly-Shia capital to try to halt a surge in murderous bomb and gun attacks by rival sectarian gangs.

Just southeast of the capital, police found at least 12 civilian corpses, most of them kidnapping victims who had been tortured and shot.

At least 12 people were hurt in Tayaran Square market in the center of the city when a bomb exploded, indiscriminately targeting civilians. A second attack in a northern suburb hurt three police and three bystanders.

Such attacks have in recent weeks pushed the capital into chaos and seriously damaged the authority of Prime Minister Nuri al-Maliki's embattled government, a unity coalition of the main Sunni and Shia factions.

The government's US allies plan to bolster homegrown attempts to regain control, in part by deploying an additional brigade of combat troops equipped with Stryker armoured cars into the city and by stepping up raids.

But fighting has also continued to rage outside Baghdad.

On Saturday, six people were killed in various attacks around Baquba, including a police officer and a soldier, while three more civilians were kidnapped just outside the restive northern city.

A Katyusha rocket was fired into deposed Iraqi leader Saddam Hussein's hometown, Tikrit, killing a woman. Four more civilians were killed in the ethnically-mixed oil city of Kirkuk when a bomb went off by a petrol station.

Police also reported that a former official of the Baath party who worked as a teacher in Nasiriyah, deep in southern Iraq, was shot dead by gunmen.

Another civilian was shot dead in his pickup truck in Samarra, north of the capital, according to police, who could not suggest a motive for his slaying.

The ongoing violence is a sign of the success of anti-US insurgents such as those linked to the al-Qaeda extremist network in poisoning relations between Iraq's Shia majority and formerly politically-dominant Sunni.

In February, suspected Sunni extremists blew up a revered Shia shrine in the city of Sammara, triggering tit-for-tat reprisals which quickly became a self-perpetuating dirty war in Baghdad and religiously-mixed provinces.

Shia militias -- which rejoiced at Saddam's fall and have close ties with the biggest parties in Maliki's government -- stand accused of taking part in sectarian massacres.

Shia leader Abdelaziz Hakim said Friday that Iraqis should handle their own security without interference from others, a veiled reference to US forces.