Kabul: An Afghan soldier shot and killed a NATO service member in southern Afghanistan, officials said on Wednesday, in what the international coalition described as an attack and an Afghan commander called an accident.

It was the sixth report since December 26 of an Afghan soldier or an insurgent posing as one turning his weapon on the international troops working to train the Afghan security forces.

The string of attacks has raised concerns about relations between Afghan troops and their foreign allies.

International forces and the Afghan army disagreed on exactly what happened in the latest killing.

Lt Col Jimmie Cummings, a spokesman for the international military force, said Afghan soldiers detained the gunman after he turned his weapon on a NATO service member and killed him last night. The coalition gave no other details.

Afghan National Army commander Sayed Malluk confirmed the killing, which he said happened during a night watch on a base in Helmand province's Marjah district.

But he said the Afghan soldier, who has been in the army for more than two years, told investigators the shooting was an accident.

"It wasn't intentional. It was a mistake," Malluk said. He said an investigation has been opened.

After an Afghan soldier killed four French soldiers on January 20, French President Nicolas Sarkozy announced that France would speed up the exit of its troops from Afghanistan and that it would ask NATO to hand over all combat operations to Afghan forces in 2013 instead of by the end of 2014.

Sarkozy at the time said the attacker was a Taliban infiltrator, but Afghan and NATO investigators have said it was too early in the investigation to know his motivation.

The rising number of attacks by supposed friendly Afghan forces has prompted speculation that Taliban insurgents or sympathisers may be infiltrating national army and police as they rapidly expand to meet a 2014 target for Afghan forces to take over security and most international troops to leave.