President Nicolas Sarkozy has warned he may accelerate the French withdrawal from Afghanistan after an Afghan soldier shot dead four unarmed French troops during a sports session inside a base.

Sarkozy suspended French military training and joint combat operations with Afghan troops, and sent Defence Minister Gerard Longuet to probe Friday's attack in which at least 15 French soldiers were also wounded, eight seriously.

"The French were finishing their sports' session," a security official in Afghanistan told AFP.

"The soldiers had no protection. They couldn't defend themselves. He opened fire on the group. Then they overcame him."

Longuet described the attack as "murder".

"They were not armed. They were literally murdered by an Afghan soldier. We don't yet know if it was a Taliban who infiltrated or if it was someone who decided to act for reasons as yet unknown," Longuet said.

The French role in the NATO-led mission in Afghanistan was already deeply unpopular at home and -- less than 100 days before presidential elections -- Sarkozy appeared to be preparing the ground for a rapid withdrawal.

Sarkozy's Socialist opponent Francois Hollande, who according to opinion polls is the most likely victor in the poll, said that if elected he would order the 3,600-strong contingent home by the end of the year.

France was already concentrating on training Afghan forces and accompanying them in combat rather than leading its own offensives against Taliban rebels, so Friday's suspension of operations effectively halted its core role.

"The French army stands alongside its allies but we cannot accept that a single one of our soldiers be wounded or killed by our allies, it's unacceptable," a clearly tired and angry Sarkozy told diplomats.

"If security conditions are not clearly established, then the question of an early return of the French army will be asked," he warned.

In Washington, US Secretary of State Hillary Clinton expressed her sympathy with the French for their loss during a news conference with German Foreign Minister Guido Westerwelle.

But she added: "We are in close contact with our French colleagues and we have no reason to believe that France will do anything other than continue to be part of the very carefully considered transition process as we look at our exit as previously agreed upon in Lisbon."

The Pentagon said it was up to France alone to decide.

"Their contributions are theirs to determine and theirs to amend as they see fit," Pentagon spokesman Navy Captain John Kirby said, calling the French "great allies and great friends."

Australian Foreign Minister Kevin Rudd said, as far as he was aware, France remained committed to the NATO-led mission, following meetings with his counterpart Alain Juppe.

"As to the impact of these recent tragic losses by the French in Afghanistan, then that of course is a matter for the French government.

"But I detected no erosion of resolve on the part of the French in my meetings with the foreign minister in Paris," Rudd said.

The attack came as the New York Times published details from a classified coalition report that said attacks from Afghan troops were a growing threat.

"Lethal altercations are clearly not rare or isolated; they reflect a rapidly growing systemic homicide threat (the magnitude of which may be unprecedented between 'allies' in modern military history)," it said.

NATO played down the threat, insisting attacks were rare.

"Such tragic incidents are terrible and grab headlines but they are isolated," Secretary General Anders Fogh Rasmussen said, noting that 130,000 NATO-led international troops were serving alongside 300,000 Afghans.

But the secret report said such NATO denials risk seeming "disingenuous, if not profoundly intellectually dishonest," the Times wrote.

Coalition officials also played down the role of Taliban infiltrators in such incidents, arguing that in many cases they were the result of a culture clash or difficult personal relations between Afghan and foreign forces.

Some Afghans have been angered by abuses by coalition forces, such as that revealed last week when a video showed US Marines urinating on the corpses of dead Afghans and joking about it.

John Allen, the commander of the NATO-led troops, promised to work closely with France on a "full and complete investigation."

French Prime Minister Francois Fillon called the situation "intolerable" adding that the defence minister had been sent to Afghanistan to see how security for the French troops could be improved.

French troops have fanned out around their base in the eastern province of Kapisa and are not allowing any Afghan soldiers to approach, a security source told AFP.

The French force currently in Afghanistan will be reduced to 3,000 by late 2012, with 200 due to leave in March. NATO is due to hand security over to Afghan forces before withdrawing all its combat troops by the end of 2014.

Afghan President Hamid Karzai sent his condolences to the French people over the deaths, saying relations between the two countries had "always been based on honesty, which makes Afghans happy."

The latest deaths brought to 82 the number of French soldiers killed in Afghanistan since French forces deployed there at the end of 2001.

Last month, two soldiers of the French Foreign Legion serving in Afghanistan were shot dead by a man wearing an Afghan army uniform during a mission in Kapisa, site of the main French base in Afghanistan.

The Taliban claimed responsibility for that attack.

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