The US military said today it was conducting a "thorough investigation" of an air strike by its attack helicopters north of Baghdad last night that killed nine children and six women.
The civilian death toll was one of the largest acknowledged by US forces from an air strike since former president Saddam Hussein was toppled in 2003.
The incident, on the eve of the Eid al-Fitr holiday, is likely to reignite tensions between Washington and Baghdad, which has repeatedly criticised US forces over the number of Iraqi civilians killed in military operations.
There was no immediate response from the Iraqi government, which was shut down for the Muslim holiday.
US military spokesman Major Brad Leighton
The 15 were killed during an operation targeting senior leaders of al-Qaeda in the Lake Thar Thar area 80 kilometres northwest of the Iraqi capital. Nineteen suspected insurgents were also killed, the US military said.
"We are doing a thorough investigation to understand the events that surround the incident, especially since there was a significant loss of civilian life," a US military spokesman said.
Under the military's rules of engagement, soldiers were permitted to take measures to protect themselves if they came under fire, he said.
In yesterday's operation, the US military said intelligence reports and surveillance indicated that senior al-Qaeda figures were meeting in the Thar Thar area. An initial air strike on the site killed four insurgents.
Some suspects were then seen leaving the site and travelling to an area south of the region, where US forces were fired upon from gunmen in a building.
The attack came the same day the United Nations called on US forces to investigate the killing of civilians in air strikes and raids by ground forces and make the findings public.
In its report, covering the April to June period, the United Nations Mission in Iraq said it had recorded a number of incidents in which 88 civilians were reported to have been killed in US air strikes.