A U.S. airstrike killed at least nine "local security personnel aligned with Afghan Government forces," a spokesman for the U.S. military confirmed late Friday.

U.S. forces apparently mistook the police for the Taliban militants both the U.S. and Afghan forces were fighting.

The airstrike took place in the Gereshk district of Helmand province, where U.S. and Afghan forces have been locked in fierce fighting with the Taliban. Local media reports put the number of dead at 17. A U.S. military statement said that an investigation would be conducted "to determine the specific circumstances that led to this incident."

Helmand's governor, Hayatullah Hayat, said two commanders of the national police were among those killed, and that many bodies were still under rubble, making an exact toll hard to determine.

According to Hayat, the Taliban had recently seized a number of police posts and captured Humvees stationed there, making it easier for aircraft to confuse the two groups.

"The police had recaptured a post from the enemy and it was hit by mistake with the belief that the Taliban fighters were still dug in there after they took the post on Thursday," said Hayat, who was reached by phone. He claimed that 70 Taliban fighters had also been killed in the offensive, which took place around midday Friday.

A U.S. defense official, speaking on the condition of anonymity because official details about the strike had yet to be released, said that U.S. Marines in Helmand had helped coordinate the strike with Afghan security forces. The official said that the area had recently been taken over by the Taliban and that their Afghan counterparts indicated that the positions were not friendly or manned by government forces.

When aircraft were overhead, they saw a group of individuals not wearing uniforms and firing in what appeared to be the wrong direction, the official added.

The U.S. air war in Afghanistan has returned to a level on intensity not seen since the troop surge in 2012, when 80,000 U.S. forces were spread across the country, going on patrols and engaging the Taliban on a daily basis.

Data released this month by the Air Force says that as of June 30, U.S. and coalition aircraft had dropped or expended 1,634 munitions in Afghanistan during the first six months of 2017. By comparison, that figure was 298 in 2015 and 545 in 2016.

The sharp upward trend is due in part to new powers given to commanders on the ground last June by then-President Barack Obama. U.S. forces had previously only been able to conduct airstrikes when troops, Afghan or American, were deemed to be directly under threat.

U.S. Navy Capt. Bill Salvin, a spokesman for U.S. forces in Afghanistan, said this week that 70 percent of all airstrikes in the country were directed against the Taliban, while about 20 percent were part of counterterror operations, such as going after the Islamic State's Afghan affiliate.