President Nicolas Sarkozy warned Friday 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 an attack in which at least 15 French soldiers were also wounded, eight seriously. 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 and -- according to opinion polls -- the most likely victor in the poll, Francois Hollande, said that if he is 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, the Pentagon said it was up to France alone to decide whether to accelerate the withdrawal. \"We mourn for their losses today, but those are decisions that only the French government and the French people can make,\" Navy Captain John Kirby, a Pentagon spokesman, said of Sarkozy\'s remarks. \"Their contributions are theirs to determine and theirs to amend as they see fit,\" Kirby said, calling the French \"great allies and great friends.\" The attack came as the New York Times published details from a classified coalition report that said attacks from Afghan troops are 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 are serving alongside 300,000 Afghans. But the secret report, according to the Times, said such NATO denials risk seeming \"disingenuous, if not profoundly intellectually dishonest.\" Coalition officials also play down the role of Taliban infiltrators in such incidents, arguing that in many cases they are 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 was released showing US Marines urinating on the corpses of dead Afghans and joking about it. \"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. Foreign Minister Alain Juppe said France would await a report from Longuet and military chief of staff Admiral Edouard Guillaud on their return from Afghanistan before taking any decision on an early pull-out. If Sarkozy is not satisfied the threat of further attacks is contained, he will study other options \"including the acceleration of a complete withdrawal of our contingent set for the end of 2013,\" Juppe said. French troops have fanned out around their base in the eastern province 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 president is saddened at the incident and expresses his deep sympathy and condolences to the president and people of France and the victims\' families,\" his office said in a statement. 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. In April last year, eight US soldiers were killed in a shooting at a military airport in Kabul, but a Pentagon report this month said the killings were the actions of a disturbed Afghan military officer who acted alone.