* NATO air strike hits Afghan army outpost after clash
* Interpreter kills two U.S. soldiers
* NATO kills two civilians in separate incident
By Asif Andalib
SALAR, Afghanistan, Jan 30 (Reuters) - NATO troops clashed with their Afghan allies in a so-called "friendly fire" incident on Saturday, calling in air strikes that killed four Afghan soldiers and stoked anger among villagers.
The clashes took place hours after an apparently disgruntled interpreter shot dead two U.S. soldiers at a nearby base. The incidents, although not apparently linked, highlighted the fraught relationship between Western forces and their Afghan hosts.
NATO and Afghan officials tried to head off tension by announcing a joint investigation into how their troops ended up battling each other in Wardak province, southwest of Kabul.
"Four army soldiers were killed and six wounded when a foreign forces air strike hit their post," said Shahedullah Shahed, spokesman for Wardak's governor. "We don't know why it happened, but it is deeply regrettable."
He said the strike had targeted an Afghan Army outpost that had been newly established. Foreign forces and Afghan troops were both separately conducting overnight operations when they started shooting at each other, he said.
The NATO-led International Security Assistance Force (ISAF) said its troops had come under fire and called in air strikes, without realising they were engaging Afghan security forces.
"Initial post-operational reports indicate the small arms fire originated from an Afghan National Army (ANA) combat outpost and the subsequent air support called in by the joint force likely killed at least four ANA soldiers," a statement said.
"We work extremely hard to coordinate and synchronise our operations," said Canadian Brigadier-General Eric Tremblay, the force's main spokesman. "This is a regrettable incident and our thoughts go out to the families of those killed and wounded."
Hours earlier, an interpreter opened fire at a base in the same province, shooting dead two U.S. soldiers before he was killed, two U.S. military officials said, under condition they not be named because details had yet to be officially released.
"Initial indications are this was a case of a disgruntled employee" rather than an insurgent attack, one of the U.S. officials said. An Afghan provincial official confirmed the account, saying the interpreter had argued with troops over pay.
In a separate incident in nearby Ghazni province, ISAF said on Saturday its troops had shot dead two Afghan civilians and wounded a third when they failed to heed warnings to stop the vehicle in which they were travelling. Similar shootings have led to demonstrations against Western troops in recent weeks.
"Friendly fire" incidents between Afghan and foreign forces and killing of Afghan civilians are among the biggest sources of tension between the Afghan government and the Western troops fighting to protect it.
"As you can see, they dropped bombs on the outpost. It was the Americans of course. Who else can bomb us?" an angry village elder told Reuters television in the town of Salar, gesturing towards the sky above the site of the "friendly fire" incident.
The NATO-led force, which is about two-thirds American, did not identify the nationality of the troops involved.
The Afghan Defence Ministry called for a court martial for any troops found responsible for the clash.
"The soldiers involved in the horrific incident must be dealt with according to martial law, without any hesitation, so that they receive punishment for their action," the ministry said.
Western forces are also concerned about increasing numbers of attacks from the Afghans they work with.
In November, an Afghan policeman killed five British soldiers at a checkpoint in southern Afghanistan. In December, an Afghan soldier killed a U.S. service member and wounded two Italian soldiers when he opened fire at an army base in the west.
Later that month, a Jordanian double agent wearing a suicide vest killed five CIA staff, two CIA contractors and a Jordanian intelligence officer, the deadliest attack on the CIA in decades.
The United Nations says ISAF has managed to reduce the number of civilians killed since its commander, U.S. General Stanley McChrystal, issued guidelines last year to curb such deaths. (Additional reporting by Sher Ahmad in GHAZNI and Hamid Shalizi and Peter Graff in KABUL; writing by Peter Graff; editing by Janet Lawrence) (For more Reuters coverage of Afghanistan and Pakistan, see: here)