The fresh unrest in Samarra, 95 km north of Baghdad, started just before 11:00 (07:00 GMT) on Thursday when the Iraqi national guard headquarters at the entrance of the city was attacked.
A car bomb was detonated followed by a barrage of mortar fire levelling the military headquarters and killing five US soldiers and one Iraqi guardsman, the US military said.
American troops responded with helicopters attacks and fanned out through the city in tanks.
Another US soldier was killed in Baghdad in small arms fire, the military said on Friday. The latest deaths raised the number of dead US troops in Iraq to at least 650.
The violence also killed three civilians, medical officials said. As many as 44 people were wounded, including 20 US soldiers and four Iraqi guardsmen, the military and hospital officials said.
"There was a mortar attack on the Iraqi national guard headquarters in Samarra. They fired four mortar rounds striking and collapsing the building, used by the national guard and 1st Infantry Division (ID) soldiers"
Major Neal O'Brien,
"There was a mortar attack on the Iraqi national guard headquarters in Samarra. They fired four mortar rounds striking and collapsing the building, used by the national guard and 1st Infantry Division (ID) soldiers," said spokesman Major Neal O'Brien.
One witness, Khalid Salih, said the gate of the headquarters building shared by US forces and their Iraqi National Guard allies was open when a sport utility vehicle with a car bomb drove in.
"I saw a GMC entering into the base and immediately exploding," he said.
US patroller attacked
Also Thursday, an American patrol was attacked with small arms fire in Baghdad, killing one US soldier, the US military command said on Friday. US forces detained two in the attack and three others in a separate incident.
In another attack, gunmen along the road from Samarra to Balad strafed a truck, killing the two Turkish truck drivers inside and causing the vehicle to flip over, witnesses said.
Elsewhere, four large explosions were heard at an Iraqi base in the town of Mishahda, 40 km north of Baghdad. Volleys of gunfire broke out immediately afterward. US military officials had no immediate comment.
Former Baathist killed
A former Baath party official was killed on Thursday when a bomb hidden in his car exploded outside the Baghdad rope factory that he owned, police said.
An ex-Baath party official was
killed in a separate explosion
Iraqi police and US troops sealed off the area where the man's charred remains lay by the burnt wreckage of his vehicle, destroyed when the booby trap detonated.
"He was getting into his car when it exploded," said Anmar Yassine, the senior police officer at the scene, a street in an industrial district in the south of the capital.
The victim, Ali Abbas, had been the treasurer of a regional committee of ousted President Saddam Hussein's Baath party.
In Baghdad, national guardsmen were ambushed for the second time in less than 24 hours, killing an Iraqi soldier in a street battle, the US military said on Thursday.
"One guardsman was killed and 12 wounded" in the fighting which broke out around 10 pm (18:00 GMT) on Wednesday, a military spokesman said.
The US army was called in to reinforce the Iraqi national guardsmen and help beat back the assailants, the military said.
US medical evacuation helicopter
flies over Baghdad
"The 1st Cavalry Division detained nine anti-Iraqi forces and killed a rocket-propelled grenade gunner," the spokesman said.
The shootout came after an ambush on national guardsmen on Wednesday morning in central Baghdad, killing two guardsmen and wounding 21 others.
But four Iraqis were injured earlier in the day when clashes erupted between US patrol and unidentified armed assailants in the Abu Ghraib district west of Baghdad.
Aljazeera's correspondent said the aftermath of the fighting resulted in burnt out and damaged shops.
The area was then sealed off as US troops searched for the assailants.