At least 18 people were killed on Thursday in a barrel bomb attack on a rebel-held area in northern Aleppo in Syria, a monitoring group said.The crude bomb, allegedly dropped from a helicopter, hit the neighbourhood of Qadi Askar in Aleppo, Xinhua news agency reported citing the London-based Syrian Observatory for Human Rights.The observatory, which says it relies on a network of activists on the ground inside Syria, said the toll would likely rise due to reports of people missing or critically injured.The attack came one day after an array of jihadi groups staged a blast near the Aviation Intelligence Headquarters in the government-controlled section of Aleppo, killing at least 20 government troops and allied fighters.