Beirut - The Islamic State group on Sunday repelled an advance by US-backed forces on one of its main bastions in northern Syria, seizing back territory it had previously lost, Syrian activists and the extremist group said.The group said its fighters infiltrated villages and mountains near the ISIS-held town of Manbij that were seized last month by the Syrian Democratic Forces, a Kurdish-dominated group aided by US special forces that also includes Arab fighters.In a statement carried by the ISIS-run Aamaq news agency, the group said a fighter driving a car packed with explosives struck a gathering of Kurds amid clashes in the northwestern part of Manbij.Manbij, which lies on a key supply line from Turkey to the group's de facto capital of Raqqa, has been encircled by the SDF for weeks. Heavy clashes were taking place inside.The Syrian Observatory for Human Rights also reported the ISIS counteroffensive. The Britain-based group, which relies on a network of activists on the ground, reported heavy clashes accompanied by US-led airstrikes and a series of explosions that shook the town, which it attributed to ISIS suicide operations.Humanitarian aidMeanwhile, the Syrian Arab Red Crescent said its convoy came under fire while on its way back from a mission to deliver humanitarian aid to a rebel-held suburb of the Syrian capital, and that one of its staffers was "hit". In a statement issued on Sunday, it said one of the cars in the convoy was shot at, adding that the car was clearly marked.The convoy delivered aid on Saturday to the besieged areas of Zamalka, Erbeen and west Harasta in the eastern suburb known as Eastern Ghouta.Also on Sunday, Syrian President Bashar Assad issued a presidential decree forming a new government following parliament elections held in April.Foreign Minister Walid al-Moallem, Interior Minister Ibrahim al-Shaar and Defence Minister Fahd Jassem al-Freij kept their posts. Adib Mayalleh, the former central bank governor, was named minister of economy and foreign trade.April's elections were held in government-controlled parts of the country and rejected by the opposition and its Western backers as a sham.Earlier this month, Assad appointed Electricity Minister Imad Khamis, a member of his Baath party, as prime minister-designate tasked with forming a government.