The governor of a province in northeastern Afghanistan said Monday a NATO airstrike killed four police officers at a checkpoint in the remote, mountainous region.

Nuristanis walk along an alpine track in the remote, mountainous Afghan province. Nuristan's governor says a NATO airstrike accidentally killed four police officers. Kim Jae-Hwan/AFP/Getty

Jamaluddin Badar said the strike took place late Sunday in the Wama district of the province of Nuristan, a lawless, rugged area near the border with Pakistan. He said coalition forces detained 12 police officers following the airstrike.

NATO said it was aware of the reports of a friendly fire incident and was investigating.

Botched airstrikes and night raids are the leading cause of tension between the U.S.-led occupying forces and the Afghan government. President Hamid Karzai has demanded that the coalition take steps to ensure that airstrikes do not cause accidental deaths. The United Nations said in a recent report that airstrikes conducted by U.S.-led foreign troops remain the leading cause of civilian deaths by pro-government forces.

Badar condemned the incident, which he said occurred "while the flag of Afghanistan flew from the checkpoint and all police were in uniform."

"The repetition of such mistakes will have a bad effect on the police ranks in the province," he warned in a statement. It is unclear how many soldiers NATO countries have in the province, where security is provided largely by a small force of Afghan police.

Although Taliban-caused civilian deaths outnumber NATO's by more than four-to-one, Adm. Mike Mullen, chairman of the U.S. Joint Chiefs of Staff, told reporters in Kabul on Sunday that reducing them was a top priority for international forces.

"We are working hard to completely eliminate these casualties," he said.