The United States said Friday that an airstrike in Afghanistan's southern Helmand province killed an unspecified number of Afghan security forces. The provincial governor confirmed the deaths, saying at least two commanders were among those killed but the final death count was not immediately known.

The Department of Defense said the U.S. was supporting Afghan troops, who were carrying out an operation against the Taliban in the province's Gereshk district, when the bombing took place.

"We would like to express our deepest condolences to the families affected by this unfortunate incident," the statement said. "During a U.S. supported ANDSF (Afghan National Defense Security Force) operation, aerial fires resulted in the deaths of the friendly Afghan forces who were gathered in a compound."

Helmand provincial Governor Hayatullah Hayat said the NATO airstrike hit on Friday and that at least two Afghan commanders died.

He added that the death toll was expected to rise further because several security force personnel were known to have been in the compound at the time. Hayat also said many of the security forces were not in uniform, which may have caused some confusion about their identities.

The incident was still under investigation, he said.

While much of Helmand province is under the control of the Taliban, the Afghan national security forces have been waging fierce battles to retake territory. NATO and U.S. troops are in Helmand to assist the Afghan troops.

Elsewhere in Afghanistan an insurgent attack in western Ghor province killed four police officers and wounded seven.

Mohammad Mustafa Moseni, Ghor's provincial police chief, said Friday that the Taliban attacked police compounds in the provincial districts of Taywara and Pasaband, considered key to the security of the provincial capital.

The attacks, which occurred overnight, resulted in a four-hour gunbattle in which 24 Taliban were killed, said Moseni.

In northern Baghlan province, fighting has raged for days on a key highway linking several northern provinces to the capital of Kabul. Hospital officials said eight local police and two civilians died earlier this week in one incident.

Deputy provincial police chief Said Amir Gul Hussainkhil said the fighting is continuing but it is sporadic.

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Associated Press writer Robert Burns in Washington contributed to this report