A suicide car bomber Tuesday killed 12 people, nine of them foreigners, officials said, in an early-morning attack claimed by a militant group which said it struck to avenge an anti-Islam film. The attack on a highway leading to Kabul international airport was the second suicide strike in the heavily fortified city in 10 days, renewing questions about stability as NATO accelerates a troop withdrawal and hands over to Afghan forces by the end of 2014. Afghan insurgent group Hezb-i-Islami claimed responsibility for the blast, saying it was carried out by a woman to avenge the "Innocence of Muslims" film, deemed insulting to Islam. The claim was made by spokesman Zubair Sidiqi in a telephone call to AFP from an undisclosed location. It is extremely rare for the faction to claim a suicide attack in Afghanistan. It is also rare for women to carry out suicide attacks. Hezb-i-Islami is Afghanistan's second-biggest insurgent group after the Taliban and is led by Gulbuddin Hekmatyar, a former prime minister. An AFP photographer saw at least six bodies lying among the wreckage of a gutted minivan, and another vehicle destroyed by flames still burning in the middle of the highway, with debris flung all around. "At around 6:45 am (0215 GMT) a suicide bomber using a sedan blew himself up along the airport road in District 15. As a result, nine workers of a foreign company and three Afghan civilians are dead, and two police are wounded," police said in a statement. An Afghan and a Western security official said nine foreigners were killed. "The foreigners were from a private company working at the airport," the Afghan official told AFP, speaking on condition of anonymity. Kabul police chief Mohammad Ayoub Salangi told AFP the bomber blew himself up alongside a minivan, carrying foreigners. The attack came a day after protests turned violent for the first time in Afghanistan over "Innocence of Muslims", as hundreds of angry men hurled stones at a US military base, clashed with police and shouted "Death to America". The movie, believed to have been produced by a small group of extremist Christians, has sparked a week of furious protests outside US embassies and other American symbols in at least 20 countries, killing 19 people. A spokesman for NATO's US-led International Security Assistance Force (ISAF) confirmed an explosion near the airport, but said there was no current report of casualties among its personnel. NATO, which is helping the Afghan government fight a Taliban-led insurgency now in its 11th year, is gradually withdrawing its 112,600 remaining troops. The United States, which leads the force, has 77,000 US troops in the country. The attack took place on the eight-lane highway in front of a wedding hall, which would have been deserted at the time. Witnesses said there was smoke spewing into the sky and a heavy police deployment at the scene of the attack, contributing to a major traffic snarl-up on the busy road. On September 8, a suicide bomber killed at least six people, most of them children, outside ISAF headquarters in Kabul in an attack that the Taliban claimed targeted the CIA to avenge US moves to blacklist its Haqqani network as a terrorist organisation. Tuesday's attack came after a devastating few days for NATO in which six of its soldiers were shot dead by suspected Afgan police, the Taliban destroyed six US fighter jets in an unprecedented assault on a major base in the south and one of its air strikes killed eight Afghan women. NATO insists the insurgency in Afghanistan is on the back foot, with Afghan forces taking the lead for security over 75 percent of the population, as part of the phased departure of most Western troops. But concerns are growing about how to halt so-called insider attacks, in which Afghans turn their weapons against their NATO colleagues. At least 51 Western soldiers have been killed in 36 such incidents so far this year alone. US Defense Secretary Leon Panetta on Monday sought to downplay such fears, calling the attacks a "last-gasp" tactic from Taliban who have lost ground in the last two years since a surge of NATO troops, now being withdrawn.