(Recasts with U.N. reaction)

By Ross Colvin

BAGHDAD, Oct 12 (Reuters) - The United Nations mission in Iraq urged U.S. forces on Friday to pursue a "vigorous" probe into an air strike that killed 15 women and children and said its findings must be made public so that lessons can be learned.

It said the safety of civilians should be a top priority during military operations. In a human rights report published on Thursday, the same day as the attack, the mission highlighted the number of Iraqi civilians killed in recent U.S. air strikes.

The civilian death toll in Thursday's operation was one of the largest acknowledged by U.S. forces from an air strike since former president Saddam Hussein was toppled in 2003.

"Civilians are getting caught far too often between warring combatants," said U.N. mission spokesman Said Arikat. "We understand the security concerns, but we also hope that every possible safety measure is taken not to harm any civilians.

The U.S. military said it was conducting a "thorough investigation" into the strike by its attack helicopters on a suspected meeting of senior al Qaeda leaders north of Baghdad on Thursday that killed nine children, six women and 19 insurgents.

"In every instance we take as many precautions as possible to ensure innocent lives are not at risk," military spokesman Rear Admiral Greg Smith said.

"We are committed to working with affected families and taking care of their needs."

The U.S. military says insurgents often deliberately hide among civilians and previous air strikes on suspected militant hideouts have resulted in civilian deaths.

In its human rights report, covering the April to June period, the United Nations Mission in Iraq said it had recorded a number of incidents in which a total of 88 civilians were reported to have been killed in U.S. air strikes.

Thursday's 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.

"We certainly hope that they do pursue this vigorously and that these findings will be made public and lessons learned," Arikat told Reuters.

TARGETING AL QAEDA

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.

Smith said imagery from aerial drones and the helicopters was being studied to get a better understanding of how events unfolded. Under the military's rules of engagement, soldiers were permitted to take measures to protect themselves if they came under fire.

The U.S. military said in a statement that intelligence reports and surveillance had indicated 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. A second air strike was called in.

"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 military statement said.

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