With more troops killed, April became the deadliest month for American forces since they set foot in Iraq.
Elsewhere, a 2,500-strong U.S. force massed on the outskirts of the Shiite holy Najaf for a showdown with a radical cleric, raising fears of a U.S. attack on the city that would outrage the nation's relatively pro-U.S. Shiite majority.
An envoy for the cleric said Muqtada al-Sadr has asked him to convey a set of proposals to U.S. officials, according to CBS News Reporter Lisa Barron.
With the announcement of the deaths of four more Marines, at least 87 troops have been reported killed in action in less than two weeks. Previously, November had seen the most deaths, 82. Roughly 680 U.S. troops have died in Iraq.
Some 880 Iraqis have been killed this month. Among those are more than 600 Iraqis — mostly civilians — killed in Fallujah, according to the city hospital's director.
In other developments:
A U.S. Cobra attack helicopter fired rockets and heavy machine-guns before dawn Wednesday at gunmen gathered on the northern edge of the city of Fallujah. Rocket-propelled grenades arched up from the ground toward the helicopter and a second gunship providing support, but none apparently hit the gunships.
Early Wednesday, A-130 gunships pounded a row of buildings from which Marines say ambushes have repeatedly been launched in a residential area of the city.
A day earlier, Marines came under two heavy ambushes, the best coordinated and largest guerrilla operations in days, said Capt. James Edge. Two Marines were killed Tuesday and two Monday, the military announced.
A force of 20 insurgents attacked a Marine position in a residential neighborhood, then damaged an armored vehicle that came to support it, Edge said.
A fierce battle followed to extract the vehicle as F-15s overhead fired on gunmen. Early Wednesday, A-130 gunships fired on a row of buildings in the area from which ambushes have repeatedly been launched, Edge said.
Outside the city, an MH-53 Pave Low helicopter — used to ferry special operations soldiers and large enough to carry 38 troops plus a crew of six — was hit by ground fire early Tuesday. A Marine team that came to secure it was ambushed and suffered casualties.
The Marines called a halt to offensive operations on Friday to allow negotiations between U.S.-allied Iraqis and Fallujah representatives in an attempt to ease the violence. Gunmen in the city called a cease-fire Sunday. But Marines have been responding to guerrilla fire — and striking gunmen who appear about to attack.
In the south, Iraqi politicians and ayatollahs tried to negotiate a solution to avert a U.S. attack on the city of Najaf, home to one of Shiite Islam's holiest shrines.
A vehemently anti-U.S. cleric, Muqtada al-Sadr, was holed up in his office in Najaf, shielded not only by gunmen but by the presence of the city's main shrine only yards away. He vowed to continue what he called "a popular revolution" to end the U.S. occupation of Iraq.
U.S. commanders vowed to kill or capture al-Sadr, though officials suggested they would give negotiations a chance.
"The target is not Najaf. The target is Muqtada al-Sadr and his militia," said Brig. Mark Kimmitt, deputy head of U.S. military operations in Iraq. "We will hunt him down and destroy him. We would prefer it not in Najaf or Karbala. We have very great respect for the shrines, for the Shiites."