Three foreign contractors have been killed when an Afghan dressed in police uniform opened fire at a police training camp in the western province of Herat, the International Security Assistance Force (ISAF) press desk has said.

"An individual wearing an Afghan National Army uniform turned his weapon against International Security Assistance Force contracted civilian employees in western Afghanistan today, killing three," the ISAF said in a statement on Saturday.

The individual who fired on the contracted civilian employees was killed following the engagement, ISAF said.

Al Jazeera's Bernard Smith, reporting from Kabul, said, "A police source had revealed that fighting had broken out on the outskirts of Herat city, near the airport and not far from the Italian base." 

"We understand that there have been NATO casualties - three foreign casualties - foreigners training police seconded by their respective countries.

"Two NATO helicopters arrived on the scene to quell what seems to have been some fighting that had broken out after the end of the fasting," our correspondent said.

The incident is currently under investigation.