The fighting comes as the U.S. military began re-examining security measures at bases across Iraq. On Wednesday, top Pentagon officials admitted that an attack that killed 22 people — mostly Americans — at a camp near Mosul was likely carried out by a suicide bomber who infiltrated the camp's dining tent as soldiers ate lunch.
In Fallujah, U.S. F-18 fighter-bombers were seen striking at targets in the city's outskirts. Tank and artillery fire was also heard.
Officials said U.S. Marines were killed but would not specify the number. Several insurgents were also killed, they said. However, military officials said three Marines were killed in the general area.
In other recent developments:
Authorities had planned on Thursday to allow the return of 2,000 residents to Fallujah — the first wave of tens of thousands who want to come back after being displaced by last month's bloody U.S.-led offensive to retake the rebel stronghold. But by the afternoon, only about 200 actually made the trip, some on foot, officials said Thursday.
Officials said the slow start was probably because people didn't know they were allowed in. More were expected after weekly Muslim prayers on Friday.
"Most of them get their information from the mosques so we think that tomorrow they'll get the word out more," said Lt. Col. Kevin Hansen, the Fallujah operations officer with the Marines' 4th Civil Affairs unit.
U.S. officials have hailed the military offensive to retake Fallujah in November as a major tactical victory. But many of the guerrillas are believed to have slipped out during the fighting and are now said to be operating across central and northern Iraq, fueling an increase in violence there.
The return of residents to the city — once with a population of 250,000 people — is a key part of attempts to restore and rebuild Fallujah. But while U.S. and Iraqi authorities organize the return, American troops have repeated clashed with pockets of resistance in the city. Only people in a small neighborhood called Andalus, a generally commercial district, were allowed to return on Thursday.
In the wake of the blast in Mosul, U.S. commanders are re-examining security procedures for every American base in the country, trying to figure out how a bomber got into their midst, reports CBS News Correspondent Kimberly Dozier.
They are also hinting there are some major operations in the offing aimed at the enemies who cost so many American lives this week. In the triangle of death south of Baghdad, U.S. troops have already carried out several operations, arresting 44 suspects Wednesday.
General Richard Myers, chairman of the Joint Chiefs of Staff, isn't saying whether authorities think the bomber worked at the base, or breached security.
But military investigators reportedly suspect that the bomber might have been an Iraqi worker who is now unaccounted for.
The Mosul base is surrounded by blast walls and barbed wire and watched by U.S. troops who search every person going in and check his identity.
A contingent of FBI bomb technicians has been deployed to help the military investigate the bombing, said an FBI official on condition of anonymity. The Baghdad-based FBI team will help identify the type of explosive and components used, which could provide forensic links to previous Iraq bombings.
The apparent sophistication of Tuesday's operation indicated the attacker probably had inside knowledge of the base's layout and the soldiers' schedule. The blast came at lunchtime.
"We always have force protection keeping their eyes out," Lt. Col. Paul Hastings, spokesman for Task Force Olympia, the main force that controls northern Iraq, said Thursday. "For somebody that wants to take his life and kill himself, its very difficult to stop those people."
At the Pentagon Wednesday, Gen. Richard Myers, the chairman of the Joint Chiefs of Staff, said a suicide bomber had apparently strapped an explosive device to his body and entered the dining hall where the blast occurred.
Early Thursday, hundreds of U.S. troops, Iraqi National Guards and Kurdish militiamen were seen in the streets of Mosul moving around in Bradley Fighting vehicles. In some eastern neighborhoods they entered homes in search of weapons. One of the city's five bridges over the Tigris River reopened Thursday, after all were blocked off by U.S. troops a day earlier.
A radical Sunni Muslim group, the Ansar al-Sunnah Army, said it carried out the suicide bombing at the base.