The so-called "friendly fire" incident occurred overnight, during a raid in Afghanistan's restive Helmand province, the US military command in Kabul said in a statement.

The officers, members of the Afghan Border Police, were killed when a US aircraft "returned fire" during the operation, which included Afghan and American special forces, a military spokesman said.

"We would like to express our deepest condolences to the families of the ABP members affected by this unfortunate incident," the statement said.

Omar Zwak, a spokesman for the Helmand governor, put the toll at three policemen killed and two wounded.

Both Afghan and American officials said they were investigating the incident.

Taliban insurgents have made widespread gains in Helmand, seizing many of the province's districts and threatening the provincial capital of Lashkar Gah.

Hundreds of US Marines recently arrived in Helmand to take over an advising mission as part of the NATO-led coalition training and assisting Afghan forces.

American troops, including special forces, also carry out separate counterterrorism missions.