Military spokesman Lieutenant-Colonel Paul Hastings said a truck bomb exploded near a US military outpost in Mosul on Wednesday, and a second bomb attack targeted a US patrol responding to the first blast.
The patrol also had to negotiate several roadside bombs and when it reached the combat outpost, it came under attack from about 50 fighters firing assault rifles, mortars and rocket-propelled grenades.
"Close air support was called in. Initial estimates are 25 enemies killed," Hastings said.
He said 15 US troops were wounded in the clashes, none of whom had returned to active duty.
Witnesses said the first attack was a bomb in a fuel truck that was detonated at a house in Mosul that had been occupied by US troops since last month, when anti-US fighters overran several police stations in the city and looted them of weapons and equipment.
Ruaa Ahmad, an independent Iraqi journalist in Mosul, said half of the building collapsed following the huge explosion.
"An eyewitness also confirmed that another car bomb exploded outside the building," she said.
The US military has acknowledged that security has deteriorated badly in Mosul - Iraq's third largest city - and says more American troops will be sent there before the 30 January elections.
In the deadliest single attack on Americans since the invasion of Iraq in 2003, an attack on a US base in Mosul killed 22 people, 18 of them US personnel, last week.
The second deadliest attack also happened in Mosul in November 2003, when two Black Hawk helicopters collided and crashed after coming under fire, killing 17 US soldiers.