Image caption Attacks on foreign troops in Afghanistan are increasing

Two US soldiers have been shot dead by a local policeman in Afghanistan, officials for the international Nato-led force in the country say.

The policeman, reportedly a member of a local defence force being trained by international troops, was also shot and killed, officials said.

The incident happened in the western province of Farah.

It is the latest in a series of so-called "green on blue" attacks by members of the Afghan security forces.

"Two US Forces-Afghanistan service members died this morning as a result of an insider threat attack in Farah province," the US-led International Security Assistance Force was quoted by Agence-France Presse news agency as saying.

"Green on blue" attacks About 130,000 ("blue") coalition troops are fighting insurgents alongside 350,000 ("green") Afghans

There have been 36 deaths to date this year, with most of the victims Americans

There were 35 such deaths in 2011

"A member of the Afghan local police turned his weapon against two USFOR-A service members. The attacker was shot and killed."

About 130,000 ("blue") coalition troops are fighting insurgents alongside 350,000 ("green") Afghans. But there is mounting concern over attacks on Nato troops by their Afghan allies.

Members of the Afghan security forces have killed at least 36 international coalition soldiers this year, in 27 incidents, officials say.

On 10 August, two separate gun attacks on Nato-led troops in southern Afghanistan left a total of six US soldiers dead.

In one an Afghan civilian employee shot three soldiers, all from the US, at a base in Helmand province.

Earlier on the same day, also in Helmand, an Afghan police officer shot three US marines after inviting them to dinner at a checkpoint.