An Afghan soldier has shot dead two US civilians and a fellow Afghan soldier during a weapons training exercise at a military camp in northern Afghanistan, officials have said.

The incident, which took place on Tuesday in a miliary camp near the city of Mazar-i-Sharif, also resulted in the death of the suspected assailant.

A foreign service member and another Afghan soldier were also wounded in the shooting.

Al Jazeera's James Bays, reporting from Kabul, the capital, said: "It was a basic warrior training course and the incident resulted in two US civilians who we believe are defence contractors involved in the training and two Afghan soldiers being killed.

"The official news release from the Nato forces and Afghan military says both sides are going to stage a joint investigation into what is the latest incident of this type.

"It is not clear what the reason for this particular attack was, whether there was some sort of row that took place before the attack, which is what we are being told by some of our Afghan army sources," he said.