By Ross Colvin

BAGHDAD, Oct 12 (Reuters) - The U.S. military said on Friday it was conducting a "thorough investigation" of an air strike by its attack helicopters north of Baghdad on Thursday night that killed nine children and six women.

The civilian death toll was one of the largest acknowledged by U.S. 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 U.S. 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.

The 15 were killed during an operation targeting senior leaders of al Qaeda in the Lake Thar Thar area 80 km (50 miles) northwest of the Iraqi capital early on Thursday night. Nineteen suspected insurgents were also killed, the U.S. 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," U.S. military spokesman Rear Admiral Gregory Smith told Reuters.

"In every instance we take as many precautions as possible to ensure innocent lives are not at risk," he said. "We are committed to working with the affected families and taking care of their needs."

Smith said imagery from aerial drones and the attack helicopters was being studied to get a better understanding of how events had unfolded.

Under the military's rules of engagement, soldiers were permitted to take measures to protect themselves if they came under fire, he said.

UN REPORT

In Thursday's operation, the U.S. 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 Lake, where U.S. forces were fired upon from gunmen in a building.

"Responding in self-defense, supporting aircraft engaged the enemy threat," the military said in a statement.

"After securing the area, the ground force assessed 15 terrorists, six women and nine children were killed, two suspects, one woman and three children were wounded, and one suspected terrorist was detained."

The attack came the same day the United Nations in its latest human rights report on Iraq called on U.S. 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 civilian were reported to have been killed in U.S. air strikes.

"We regret that civilians are hurt or killed while Coalition forces search to rid Iraq of terrorism," said another U.S. military spokesman, Major Brad Leighton.

"These terrorists chose to deliberately place innocent Iraqi women and children in danger by their actions and presence."

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