Nine US soldiers were killed and 20 were wounded in a suicide car bombing against a patrol base northeast of Baghdad, the military has said.
The attack occurred on Monday in Diyala province, a volatile area that has been the site of fierce fighting between US and Iraqi troops, Sunni insurgents and Shiite militias, according to a statement.
The nine Task Force Lightning soldiers died of injuries sustained in the blast, which also left 20 soldiers and an Iraqi civilian wounded, the military said.
Of those wounded, 15 soldiers were treated and returned to duty while five others and the Iraqi civilian were evacuated to a medical facility for further care, it added.
Identities were not released pending notification of relatives.
It was the second bold attack against a US base north of Baghdad in just over two months and was notable for its use of a suicide car bomber.
On February 19, insurgents struck a US combat post in Tarmiyah, about 50 kilometres north of Baghdad, killing two soldiers and wounding 17 in what the military called a "coordinated attack."
It began with a suicide car bombing, then gunfire on soldiers pinned down in a former Iraqi police station, where fuel storage tanks were set ablaze by the blast.
Militants have mostly used hit-and-run ambushes, roadside bombs or mortars on US troops and stayed away from direct assaults on fortified military compounds to avoid US firepower.