NATO and Afghan officials were Saturday probing reports a rogue Afghan soldier shot dead foreign troops -- said to be two US Marines -- on a base in the volatile south of the country, the alliance said.
A NATO official said that two US Marines had been killed in the incident, which took place in Helmand province late on Thursday night.
Speaking on condition of anonymity, the official told AFP the Marines had been shot by an Afghan soldier who had been on the base for two to three weeks and was now missing.
Referring to it as a "green-on-green" incident, he said the Marines "weren't shot in their beds, they must have been on guard duty."
"Rounds were fired within the FOB (forward operating base) and an Afghan soldier was found to be missing the next morning," he said.
NATO's media office did not immediately confirm the details, saying it was aware of "the incident in Helmand province".
A team from ISAF and the Afghan government "is investigating the incident," it said. No further details were available.
The incident was initially reported early Saturday by Pakistan-based Afghan news agency Afghan Islamic Press (AIP).
AIP is not generally regarded as reliable and often publishes Taliban propaganda, including exaggerated claims of battlefield operations.
It quoted a Taliban spokesman saying an Afghan soldier had "shot and killed three foreign troops at a base in Sangin district of Helmand".
"The ANA soldier opened fire on foreign troops at a base in Tamirano area close to the headquarters of Sangin last night, killing three foreign soldiers, Qari Muhammad Yousuf Ahmadi, spokesman of Taliban, told Afghan Islamic Press," the report said.
It quoted Ahmadi saying the "soldier fled the base and joined Taliban".
Defense ministry official Mohammed Azim Mujahid was quoted by the German news agency DPA saying the killings happened when an Afghan soldier mistakenly opened fire on a NATO patrol.
"The soldier didn't have any contact with the Taliban. It was only an accident," he said.
Sangin has been labelled the most dangerous area in Afghanistan's long war, with British forces fighting there from 2006 until September, when they handed over to US Marines.
The British lost more than 100 troops in Sangin, around a third of the total number of military casualties the country has suffered in Afghanistan since the current insurgency began in late 2001.
Helmand's governor Gulabuddin Mangal, during a visit to London earlier this week, said that security in the area had improved since the Marines moved in, as they were better manned and equipped than their predecessors.
Sangin has a population of around 20,000 and is a key distribution and supply point for Taliban money and men, as well as the drugs cartels that dominate the area.
Afghanistan supplies 90 percent of the world's opium, most of it produced in Helmand. The Taliban supply muscle to protect growers and distributors. Drugs money in turn helps fund the insurgency.
Incidents in which Afghan soldiers or police officers turn on Western troops are rare, but raise questions about the multi-billion-dollar international effort to train and mentor Afghanistan's security forces.
In July a renegade Afghan soldier shot and killed three British army Gurkhas and wounded several others on a base in Helmand province.
In August, an Afghan police officer killed two Spanish paramilitary police officers and a Spanish interpreter during a training session at a base in Badghis province.
On Saturday Spain's Prime Minister Jose Luis Rodriguez Zapatero made a surprise visit to Afghanistan, telling Spanish radio: "We are not here to stay but we have a firm commitment until the Afghans can guarantee their own security."
US President Barack Obama has set a deadline of July 2011 to begin drawing down combat troops as public opinion in the United States and its NATO partners turns against continued engagement in Afghanistan.
Integral to a withdrawal is the ability of Afghanistan's military and police to take over responsibility for national security.
The United States and NATO have more than 150,000 troops in Afghanistan fighting the insurgency. More than 620 foreign soldiers have died this year.