The shooter was killed in the incident in the west of the country, the International Security Assistance Force said in a statement, without giving further details or naming the nationalities of the victims.

An Afghan official who requested anonymity said he knew two Americans had been killed in the attack and they had been shot by an Afghan man in a police uniform in a military training centre near the Herat airport.

The number of so-called green-on-blue attacks, in which Afghan forces turn their weapons against their Western allies, has escalated this year.

The latest deaths would take the green-on-blue toll this year to at least 29, in 19 such incidents.

In the most recent incident, three British soldiers were shot dead by a man in an Afghan police uniform in the southern province of Helmand earlier this month.

Although Taliban insurgents claim some of the attacks, many are believed to be the result of cultural differences and antagonisms between Afghan soldiers and their Western counterparts.

Last week, an Afghan soldier was sentenced to death for killing five French soldiers in a green-on-blue attack in Kapisa province in January.

The incident, in which the soldier turned his gun on his French colleagues, prompted France to speed up its planned withdrawal of troops from Nato operations in Afghanistan.