The nation''s top military officer warned Monday that the deaths of Afghan civilians caught up in U.S. combat operations could cripple President Barack Obama''s revamped strategy for the seven-year-old war. "I believe that each time we do that, we put our strategy in jeopardy," Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said. "We cannot succeed ... in Afghanistan by killing Afghan civilians."

Mullen said additional forces and new tactics can help the United States turn a discouraging tide in Afghanistan. He said he was hopeful that "in the next 12- to 24 months, that we can stem the trends which have been going very badly in Afghanistan the last three years."

But speaking at the Brookings Institution, Mullen sounded frustrated that as the first of 21,000 U.S. reinforcements arrive, Taliban insurgents are having a seemingly easy time using America''s military prowess against it.

Mullen pointed to this month''s disputed U.S. airstrikes in Farah province, in which women and children were apparently among dozens of civilians killed. The United States says the Taliban is responsible for at least some of the deaths, but Mullen didn''t spend much time defending U.S. actions.

The May 4-5 incident is still under investigation and Mullen indicated the details may always remain murky.

Mullen refused to rule out the use of unmanned drones, which the United States uses to target insurgent hideouts in both Afghanistan and Pakistan. Witnesses in the latest incident say a drone flew overhead before the U.S. bombs fell.