Up to 12 people are reported to have been killed in a suicide bomb attack on a minibus carrying foreigners near the Afghan capital, Kabul. The attack happened on a major road leading to the international airport. Afghan insurgent group Hezb-e-Islami has claimed responsibility for the blast, which it says was in response to a recent anti-Islam video. It comes as Nato announces that it will restrict operations with Afghan forces from now on. The joint command of the International Security Assistance Force (Isaf) in Afghanistan cited a recent rise in insider attacks on Nato forces and events related to anger over the Innocence of Muslims film as the reasons behind the decision. Fifty-one Nato troops have been killed by Afghan soldiers so far this year - 15 in August alone. Joint operations will now only be conducted at battalion level - large operations involving several hundred troops - ending day-to-day operations, says the BBC's Quentin Sommerville. Some smaller missions may still take place, but will have to be evaluated by senior commanders on a case by case basis. However, the Afghan ministry of defence said it had not been formally notified of the changes, adding that senior officials from both sides were currently meeting. Video revenge Hezb-e-Islami also said the film - an amateur video produced in the US which insults the Prophet Muhammad - was the motivation for Tuesday's attack. That would make it the deadliest response to the film so far. A witness told the Associated Press news agency he was waiting for a bus to go to work at 06:45 (03:15 GMT) when he saw a small white sedan ram into the minibus. "The explosion was so powerful and loud that I could not hear anything for 10 minutes," said Abdullah Shah, a teacher. Had the blast been later in the morning, he said, with more traffic on the roads, the death toll would likely have been higher. The force of the blast hurled the bus some 50m (yards), AP reported. Reporters on the scene of the powerful suicide blast counted six bodies but officials say up to 12 people have died, most of them foreigners. The vehicle was believed to be transporting international staff working at the airport. A senior counter terrorism official in Kabul told the BBC: ''We can confirm that the suicide attack was carried out by a female suicide attacker. She is either a young girl or a woman.'' It is rare for women to carry out suicide attacks in Afghanistan. Some reports say this is the first suicide car bombing by a woman in Afghanistan, where only a very small minority of women know how to drive. The majority of the dead are thought to be foreigners, with reports saying they worked at the airport, possibly for an international courier company. The BBC's Bilal Sarwary in Kabul says that three Afghans, including a passer-by and a mechanic, were also killed in the blast. There has been no confirmation of the nationalities of the passengers. The attack came a day after hundreds of protesters threw rocks and torched police vehicles in an angry protest against the anti-Islam film in Kabul. Nato is preparing to withdraw most combat forces from Afghanistan by the end of 2014, handing over security to the Afghan national forces. The restriction of joint operations will be a major step back to this process, as Nato operations rely on close contact with Afghan security forces, correspondents say.