An anti-NATO demonstration in southern Afghanistan's Zabul province turned violent Friday, when three Afghan civilians and a police officer were shot dead.

Afghan police say they were forced to fire into the crowd after insurgents infiltrated the protest in the town of Qalat and shot and killed a police officer. Officials say three civilians were killed when the police returned fire.

More than a 100 angry demonstrators had taken to the streets in Qalat to protest an overnight NATO operation that killed three suspected insurgents.

Afghans have long complained about the coalition's use of night raids and airstrikes to target militants.

Violence has intensified in Afghanistan as foreign forces begin pulling out of the war-torn country and transferring security responsibility to their Afghan counterparts.

NATO said Friday that insurgents killed three of its service members in southern Afghanistan. No other details were given.

More than 320 international troops have been killed in Afghanistan so far this year.