AMMAN - Syria's mainstream rebel groups said on Sunday the escalating Russian-backed bombing of besieged Aleppo had rendered any peace process futile unless there was an immediate halt to fighting and aid deliveries under United Nations auspices.In a statement signed by more than 30 rebel groups including the biggest faction backed by Turkey, Gulf states and the West, they said the bombing campaign that has killed dozens in the past few days was "unprecedented" and rendered futile the U.N.-backed political process Washington and Moscow seek to revive.The statement also said rebels could not accept "Russia as a sponsor because it was a partner with the regime in its crimes against our people," saying Russian-backed Syrian forces were using napalm and chemical weapons without censure by the international community.