WASHINGTON/LASHKAR GAH, Afghanistan (Reuters) - A U.S. air strike killed an unknown number of Afghan security personnel in a so-called “friendly fire” incident in the southern province of Helmand on Friday, the U.S. military said, as fighting in the area has intensified over recent days.

There was no immediate word on the number of casualties or who they were but the U.S. military statement said they were “local security personnel aligned with Afghan government forces”, suggesting they may have been local militia forces.

“During a U.S. supported ANDSF (Afghan National Defense and Security Forces) operation, aerial fires resulted in the deaths of the friendly Afghan forces who were gathered in a compound,” the statement said.

Local people said as many as 15 local police, including two commanders, may have been killed and many more wounded in the strike in Gereshk district, north of the provincial capital Lashkar Gah, which has been the scene of heavy fighting over recent days. However there was no official confirmation.

“Our forces are at the scene and bringing dead bodies out of ruins. An accurate number of casualties and details of the incident will be announced later,” said Hayatullah Hayat, the provincial governor of Helmand.

The incident underlines the complicated security situation in Afghanistan as President Donald Trump’s administration weighs sending more troops as part of a new strategy for the region.

Fighting in Helmand, an opium-rich province where the Taliban controls much of the countryside and threatens Lashkar Gah, has picked up in the past week as the Afghan army has begun operations to push the insurgents back from the capital.

U.S. aircraft have carried out 52 air strikes in Helmand over the past five days, including 10 in Gereshk on Thursday, according to U.S. military headquarters in Kabul.

Earlier this week, Afghan security forces backed by U.S. air strikes retook Nawa district south of Lashkar Gah. On Thursday, insurgents attacked security perimeters set up around Gereshk, blowing up three captured Humvees packed with explosives.

The U.S. military said in a statement there would be an investigation into Friday’s incident.

“We would like to express our deepest condolences to the families affected by this unfortunate incident,” it said.