KABUL, Afghanistan — American-led troops and Afghan forces killed nine Afghan police yesterday, calling in airstrikes and fighting on the ground for four hours after both sides mistook the other for militants, Afghan officials said.

In a separate incident, NATO said it accidentally killed at least four Afghan civilians Saturday night. A NATO soldier also was killed in the east.

The two cases of accidental killings could further undercut popular support for the government and foreign forces operating here. President Karzai has pleaded with America and other nations fighting resurgent militants to avoid civilian casualties.

In the western province of Farah near the Iranian border, a convoy of foreign forces showed up in Anar Dara district and clashed with Afghan police, killing nine of them, provincial Deputy Governor Younus Rasuli said.

He said the foreign troops had not informed local officials they were coming, and the police thought they were enemy fighters. The two sides fought from about midnight until 4 a.m. yesterday, and the foreign forces used airstrikes, Mr. Rasuli said.

The American-led coalition said it was investigating the report. It said its forces, along with Afghan troops, had retaliated in defense against “a non-uniformed hostile force.”

“The combined patrol signaled their status as coalition forces, but continued to receive fire,” a military statement said. “Coalition forces then returned small arms fire and engaged the enemy with precision close air support.”

In eastern Paktika province, NATO’s International Security Assistance Force said it killed at least four civilians Saturday night when its troops fired two mortar rounds that landed nearly half a mile short of their target. NATO said it was investigating whether three other civilians also were killed in the Barmal district.