At least 15 Syrian civilians were killed in the past 24 hours by US-led airstrikes against the city of Manbej, which is controlled by the Islamic State (IS) militant group, a monitor group reported.The victims in the countryside in Syria's northern Aleppo city were from two families, and three children were killed by the US-led anti-terror coalition airstrikes, Xinhua cited the Syrian Observatory for Human Rights as saying.The airstrikes were supporting the advance of the US-backed Syrian Democratic Forces (SDF) rebels, who are fighting to reclaim Manbej from the IS, as part of a new military offensive recently unleashed to strip the IS from their key posts in northern Syria.The British watchdog group said the SDF raided 16 villages near Manbej in the past few hours and were supported by the US coalition air fleet.