A suicide bomber rammed his car into a NATO convoy in the Afghan capital on Thursday, killing at least six people in the explosion and wounding more than 30, officials said. A Muslim militant group, Hizb-e-Islami, claimed responsibility for the early morning attack and said it had formed a special "martyrdom" unit to attack foreign troops. The announcement could mean a steep escalation for the movement, which is based in northeastern Afghanistan and which has fought against the American-led coalition but is also a fierce rival of the Taliban. NATO spokesman Lt. j.g. Quenton Roehricht said the international alliance can "confirm an explosion occurred on a coalition convoy in Kabul this morning," but provided no further details. Kabul provincial police spokesman Hashmad Stanakzi said the suicide bomber attacked at about 8 a.m. with a car packed with explosives. "The explosion was very big. It set the nearby buildings on fire," Stanakzi said. Kabul Deputy Police Chief Daud Amin said it was difficult to immediately estimate the number of dead because the blast had shredded many of the victims. "We saw two dead bodies of children on the ground," Amin said. "But the rest of the bodies were scattered in pieces around." Two Kabul hospitals reported that at least six dead and 37 wounded were brought in from the scene, city hospitals chief Kabir Amiri said, adding that the toll could increase as more hospitals report in.