PARIS: France threatened on Friday to pull out early from the NATO-led war in Afghanistan after a rogue Afghan soldier opened fire on French soldiers, killing four and wounding about 15 others.

The killings in the Taghab valley of Afghanistan's eastern Kapisa province were the latest in a series of incidents that have seen Afghan troops turn on their Western allies, damaging trust.

French President Nicolas Sarkozy said all French operations on the ground were being suspended and his defence minister was dispatched to clarify things on the ground in Afghanistan.

"If the security conditions are not clearly established then the question of an early return of French forces from Afghanistan will arise," said Sarkozy.

France has almost 4,000 troops in Afghanistan as part of the 130,000-strong NATO-led force there. French troops mainly patrol Kapisa, an often restive province in mountains near Kabul. They are due to leave by around the end of 2013.

French Foreign Minister Alain Juppe told a news conference about 15 other soldiers were wounded, eight of them seriously.

NATO has been rapidly expanding the size of the Afghan security forces so that they will be able to take over all responsibility for security by the time Western combat forces leave in 2014.

Previous incidents in which Western troops were killed by Afghan colleagues have been blamed either on Taliban infiltration of the Afghan military, or on stress, indiscipline and divided loyalties within the hastily trained Afghan ranks.

"It's unacceptable that our soldiers are killed by our allies," Sarkozy said.

Afghan President Hamid Karzai said in a statement: "In regard to the killing of four French soldiers in Kapisa, I would like to express my deep sadness and condolences to the families of the victims as well as to the French people."

The Taliban said that they could not confirm whether or not the killer was a Taliban member but signalled that such attacks were part of its strategy.

In an email statement to media, Taliban spokesman Zabihullah Mujahid said: "There are a number of Afghan soldiers who have an Afghan, Islamic dignity in their hearts and who have carried out a number of attacks against foreign troops."

The Taliban "has skillfully placed the Taliban inside enemy ranks who have carried out attacks, however it is not clear whether the shooter (in Kapisa) belonged to the Islamic Emirate", he said, using another name that the Taliban call themselves.

Jimmie Cummings, spokesman for the NATO-led International Security Assistance Force in Kabul, said: "There is no indication that these incidents are linked or part of any larger coordinated effort."

More than 2,500 foreign troops have died in Afghanistan since the NATO-led war began in 2001. The latest killings take the French toll to 82.

The shooting was the latest in a string of attacks by "rogue" Afghan soldiers and police on their foreign partners, or by insurgents who had infiltrated security forces.

Dozens of foreign soldiers have been killed in recent years by what NATO dubs the "insider threat", complicating coalition efforts to train Afghanistan's army and police force.

Two French Foreign Legion soldiers and one American were killed in separate episodes of so-called "green-on-blue" shootings last month, which refer to the colours of the Afghan army and the symbol of NATO. The coalition no longer releases the number of its troops killed by Afghan soldiers.

French Defence Minister Gerard Longuet said he would report back to Sarkozy by next Tuesday after his trip to Afghanistan.

Separately, six foreign soldiers were killed in a helicopter crash in southern Afghanistan on Thursday. The Taliban, which often makes exaggerated claims of military successes, said it shot the helicopter down. NATO denied this.

It was the worst crash since August last year when 30 soldiers, mostly elite U.S. navy SEAL commandos, died when their helicopter came down in eastern Afghanistan.