The new US commander in Afghanistan said he would sharply restrict the use of airstrikes here, in an effort to reduce the Afghan civilian deaths that he said were undermining the US-led mission.

In interviews over the past few days, Gen Stanley A McChrystal said the use of airstrikes during firefights would in most cases only be allowed to prevent US and other coalition troops from being overrun. Even in the cases of active firefights with Taliban forces, he said, airstrikes would be limited if the combat was taking place in populated areas — the very circumstances in which most Afghan civilian deaths have occurred. The restrictions would be especially tight in attacking houses and compounds where insurgents are believed to have taken cover.

“Air power contains the seeds of our own destruction if we do not use it responsibly,” McChrystal told group of senior officers during a video conference last week. “We can lose this fight.”

“When we shoot into a compound, that should only be for the protection of our forces,” he said. “I want everyone to understand that.”

The statements by McChrystal signaled the latest tightening of the rules for using airstrikes, which, while considered indispensable for protecting troops, have killed hundreds of civilians.