Soldiers of the US 101st Airborne Division, which controls a large part of northern Iraq, said they were checking for improvised explosive devices when they came under rocket-propelled grenade (RPG) attack.
The troops returned fire, destroying a car from which they had been attacked and killed its four occupants, Staff Sergeant Eldon Noble said on Saturday.
Iraqi paramedics pulled out charred corpses from the car, which US soldiers said was carrying RPG rounds.
Troops in and around Mosul have repeatedly come under attack in recent weeks, including a car bombing that wounded dozens of soldiers.
Elsewhere in northern Iraq, two men were killed when a bomb they were planting exploded prematurely near the refinery town of Baiji, Iraqi police said.
The explosion happened in Safra village, 20km east of Baiji, said Colonel Atallah Hassan al-Jbouri.
Meanwhile, a US soldier died following a vehicle accident in the vicinity of Baghdad on Friday, a military spokeswoman said.
"A US soldier died of injuries sustained in a vehicle accident at 17:00 GMT on 26 December in the Baghdad area," the spokeswoman said on Saturday.
With the latest death, 126 US soldiers have been killed in non-combat incidents in Iraq since President George Bush declared major hostilities over on 1 May, according to Pentagon figures.
Japanese troops in Kuwait
The first unit of Japan's air force arrived in Kuwait early on Saturday to set the stage for humanitarian operations into Iraq, airport sources said.
The 15-member unit arrived aboard a British Airways flight from London. They were all clad in civilian clothes.
In Kuwait, the Japanese airmen will be stationed at Ali Al-Salem Air Base, some 80km northwest of Kuwait City, also close to the border with Iraq.
The second contingent of the Japanese air force is due to arrive in Kuwait by mid-January, Japanese sources said.
The Japanese air force unit will operate from Kuwait to transport goods and humanitarian aid to southern Iraq as part of Japan's reconstruction programme in the war-ravaged country.