An Afghan soldier opened fire on US civilian trainers at an army base in northern Afghanistan, killing two Americans and another Afghan serviceman before being shot dead, military officials said today.
An argument started during a weapons training exercise at a firing range outside Mazar-e-Sharif and the Afghan soldier turned his gun on the Americans, the Afghan defence ministry said in a statement.
Another Afghan soldier was killed in the crossfire, and the shooter was later shot and killed.
The attacker was a "group leader" - an Afghan soldier selected to train other soldiers on the base, defence ministry spokesman General Mohammad Zahir Azimi said.
The incident comes after another Afghan soldier killed three British service members with gunfire and a rocket-propelled grenade in the dead of night earlier this month.
The soldier fled after that attack, leaving his motive unclear, but the Taliban claimed he was a militant sympathiser taken in by insurgents after the assault.
In November, an Afghan policeman killed five British soldiers at a checkpoint in Helmand.
The attacks come as the international coalition ramps up training of Afghan soldiers and police so they can take responsibility for securing the nation. US civilian contractors - many of them retired police or soldiers - often work on Afghan army bases as trainers.
The allies set an interim goal of expanding the Afghan army from 85,000 in 2009 to 134,000 troops by October 2011. The speed with which Afghan security forces are growing has raised concerns about infiltration by the Taliban and the professionalism of the forces.
Elsewhere, Nato said six Afghan police officers were beheaded by insurgents in northern Afghanistan.
The military coalition said the militants attacked a number of government buildings, including a police checkpoint, in Baghlan province's Dahanah-ye Ghori district. The insurgents overran the checkpoint and killed the officers inside by cutting off their heads.
Violence and casualties have surged in recent months in Afghanistan. June was the deadliest month for US and international forces with the deaths of 103 service members, including 60 Americans.