KABUL: An Afghan interpreter working for the US military has shot dead two American soldiers in Afghanistan, an officer with NATO forces said Saturday.

The officer, who asked not to be named, said the incident happened on Friday in Wardak province, near the capital Kabul, but could give no further details.

NATO's International Security Assistance Force (ISAF) said late on Friday that "two US servicemembers and one US employee were killed" in an incident in eastern Afghanistan that was being investigated.

An ISAF spokesman told AFP the "employee" was an American civilian but the NATO officer confirmed he was an Afghan national employed by the US military.

No other details were available, he said.

Incidents of Afghans attacking foreign troops are not common in Afghanistan, where 113,000 foreign troops are fighting a Taliban-led insurgency, with another 40,000 due to be deployed this year.

However in a dramatic attack in November a rogue Afghan policeman shot dead five British soldiers and injured several others in Nad Ali district in the volatile southern province of Helmand.

The incident raised questions about the policy of foreign troops training Afghan police and soldiers at close quarters.

The latest deaths of US soldiers brings to 44 the total number of foreign soldiers who have died in Afghanistan this year, according to the icasualties.org website, which shows 29 were Americans.

By comparison, the website says a total of 25 foreign troops were killed in January last year.

The extra deployment of foreign troops is expected to lead to a rise in casualty rates as foreign troops backing Afghan security forces bring the fight to the Taliban, notably in its southern strongholds.

The anti-Taliban effort is being backed up by what US officials call a "civilian surge", as the renewed strategy for the war includes backing military action with aid and development.

Many of those civilians -- specialists in everything from wheat cultivation to stress counselling -- are fanning out to military bases across Afghanistan, where they are likely to be vulnerable to Taliban attack.

Afghan civilians are also increasingly vulnerable to the tactics of both Taliban fighters and their foes. ISAF reported on Saturday that two Afghans had been killed after failing to stop their vehicle when ordered.

The incident happened in the southern province of Ghazni on Friday when "an ISAF force fired on a vehicle when it failed to heed several warning signs to stop," ISAF said.

"Unfortunately, two Afghan civilians were killed and one was injured by the disabling shots fired at the engine block of the vehicle as it approached at a high rate of speed," it said, adding: "A fourth civilian in the vehicle was unharmed."

Civilian deaths jumped last year to 2,412, making 2009 the deadliest year for ordinary Afghans since the US-led invasion, a UN report said earlier this month. Most deaths were caused by the Taliban.

In a friendly fire incident in Wardak province, southwest of Kabul, four Afghan soldiers were killed and another seven wounded, according to a spokesman for the provincial governor.

"Afghan soldiers and ISAF were conducting a joint operation, after which the Afghan soldiers were returning to their base in Salar, in Sayed Abad district, when clashes erupted in the dark around 3:00 am (2030 GMT)," said Shaidullah Shahid.

He said the ISAF forces also sustained casualties but he had no details and ISAF had no comment.

In the northwestern province of Badghis, eight Taliban fighters were killed when an air strike was called in during clashes between the militants and NATO forces, the deputy provincial police chief, Abdul Jabar, told AFP.