The most intense fighting on Friday took place in the restive town of Falluja west of Baghdad, which also claimed the lives of eight Iraqi civilians, including three children.
The US military later confirmed that a marine was killed in Fallujah where the occupation forces battled resistance fighters for several hours.
Earlier during the day, four Iraqi Civil Defence Corps (ICDC) personnel were killed and four wounded in heavy fighting north of Baghdad.
Three fighters too were killed in the encounter, which broke out at around 4:00 am (0100 GMT) near Tikrit, 180km from the Iraqi capital, which also involved US troops, an occupation military spokeswoman said.
Eight Iraqi civilians were killed, including three children, after running battles between US occupation forces and resistance fighters in the town of Falluja.
Among the dead was also an Iraqi cameraman working with the US television network ABC.
The death of the US marine brought to 400 the total number of US troops killed in combat since the state of the US-led war in Iraq just over a year ago.
According to hospital sources, a further 25 people were wounded in the latest clashes.
Anti-US fighting continuing
unabated in Iraq
Issam Mohammed, a doctor at the main Falluja hospital, said the wounded also included women and old people.
Several explosions echoed through the streets, which were deserted apart from ambulances and US military vehicles.
A mosque loudspeaker broadcast the call to Friday prayers, but residents had to stay huddled in their homes.
US troops turned back reporters trying to reach the town, where explosions and heavy gunfire could be heard from the outskirts.
Falluja, about 60km (37 miles) west of Baghdad, has been known for its fierce hostility to the US-led occupation.
The US military had no immediate word on the fighting in the town.
In Baghdad, at least six civilians were wounded, including a young child, in a series of blasts on Friday evening, hospital officials said.
Doctors at Baghdad's Yarmouk hospital said three men, a woman, a teenage boy and a child were brought into the emergency room shortly after the explosions. Two were seriously wounded and moved to another hospital.
There were reports of blasts in several places. It was not clear what caused them. In one residential district, blood stained the road and there was a hole from the impact of what witnesses said could have been a mortar or rocket.
"I was standing with my friends and then we heard a strong blast," Muhammad Sahib, a 16-year-old being treated in Yarmouk, said.
"Something fell on us from the sky. I don't know what it was, a missile or something else."
Fire fighters tend to a pipeline
blaze earlier this week
Meanwhile, another crude oil pipeline feeding southern Iraqi export terminals is on fire.
A British military spokesman said they understood that a pipeline valve had failed and fire broke out from the spillage.
The British army, which controls occupation forces in southern Iraq, said there were no explosives involved in the incident.
Iraqi guards on duty at Shuaiba, an area near the city of Basra, said alleged saboteurs ignited oil that leaked from the pipeline on Friday.
A pipeline network fed from southern oilfields passes through Shuaiba on the way to Gulf export terminals.
British occupation soldiers were on the scene as huge flames and clouds of smoke rose into the air.
An explosion damaged another oil pipeline in southern Iraq on Wednesday but the main crude export link to the Basra oil terminal was not affected, officials said, adding that a technical fault was likely behind the explosion.