KABUL - NATO is looking into reports that dozens of civilians died in clashes and air strikes in western Afghanistan, its top commander in the country said, but insisted that only militants were targeted.

The U.S.-led military coalition in Afghanistan has said international and Afghan troops killed 136 suspected Taliban fighters in the Zerkoh Valley of Herat province last week, in some of the deadliest fighting so far this year.

However, an investigation by Afghan officials has found that 51 civilians died, prompting President Hamid Karzai to warn that Afghans can no longer accept such losses.

Gen. Dan McNeill, commander of NATO's International Security Assistance Force, said in remarks released Friday that he was personally examining detailed battlefield reports.

"This much I can tell you about it: Only firing insurgents were targeted,'' McNeill told a group of journalists in Brussels via teleconference on Thursday, according to an official transcript of his comments.

"If there have been civilian casualties that's regrettable,'' McNeill said.

"But there is a lot of allegations and not a whole lot of substantiations. We are going into this and are looking at this thoroughly, and we will take whatever necessary actions there are'' to avoid harming civilians, he said.

Civilian deaths have deepened Afghans' distrust of international forces and of the U.S.-backed government as they try to combat a resurgent Taliban-- itself accused by human rights groups of indiscriminate attacks that often kill noncombatants.

According to an Associated Press tally, based on reports from Afghan and western officials, 151 civilians have been killed by violence in the first four months of this year, including at least 51 blamed on NATO and the U.S.-led coalition.

The figures do not include the alleged civilian fatalities in Herat, which earlier this week sparked angry anti-U.S. protests by residents.