Two car bombs hit a rebel-held post on the Syrian border with Turkey on Monday, killing at least 16 people and closing the frontier, opposition activists and fighters said. _0"> The Bab al-Hawa crossing is held by a rebel alliance called the Islamic Front, which has been fighting with the Islamic State in Iraq and the Levant (ISIL), a small but powerful affiliate of al Qaeda with a core of foreign fighters. It was not immediately clear who had planted the bombs. The attack occurred a few days after a car bomb that killed 26 in the eastern city of Jarablus and which activists blamed on ISIL. More than 1,000 rebels have died in clashes between rival groups in the last three weeks in an upsurge of internecine violence that has weakened the nearly 3-year-old armed campaign to topple President Bashar al-Assad. The anti-Assad Syrian Observatory for Human Rights, a Britain-based monitoring group, said at least six of the dead from Monday's bombing were Islamist fighters. It