In one incident, a US Marine was killed and two wounded in an attack on their convoy east of Falluja, the US occupation military said.
The fighters exploded a bomb close to the convoy and then fired rocket propelled grenades and small arms, a military spokesperson said
In the other incident, one US soldier was killed and two others wounded when a bomb was detonated near their patrol in Baquba, 65 km north of Baghdad.
The clashes brought to 399 the number of US occupation troops killed in action in Iraq since the start of the US-led invasion just over one year ago.
Iraqi resistance fighters most often attack US patrols and convoys using what are called improvised explosive devices - small amounts of explosive hidden in a soft drink can or similar disguise and wired to a simple detonator.
The roadside bombs have increasingly been followed by small arms fire or grenade attacks.
Oil well explosion
"The well is a principal producer for oil exports through the Iraqi-Turkish pipeline and for covering local market demands"
director-general, Northern Iraqi Company
Meanwhile, an explosion on Thursday set ablaze a main oil well in northern Iraq that feeds exports through Turkey, the director-general of the state-run Northern Iraqi Company (NOC) said.
"The explosion occurred at 3:30 pm (1230 GMT) because of an explosive charge planted by unknown individuals inside the well, located 75 km west of Kirkuk," said Adel Qazzaz.
"It inflicted massive damage in the well, and firefighters are having a hard time extinguishing it because the explosion occurred inside the well and not in the pipelines," he said.
Qazzaz, who said firemen would need two days to put out the fire, said "the well is a principal producer for oil exports through the Iraqi-Turkish pipeline and for covering local market demands."
Oil-rich Iraq has found it difficult to reopen its export pipelines, which have been repeatedly attacked, mainly in the north, by fighters opposed to the continuing US-led occupation.