At least 40 militants were killed on Thursday by Syrian aerial bombardment on rebel-held areas in the northern city of Aleppo, a monitor group reported.Three commanders were among those killed when a Syrian aircraft dropped a crude barrel bomb and fired a missile into the militants' position in the Al Sha'ar neighbourhood in Aleppo, said the Syrian Observatory for Human Rights, according to Xinhua.The London-based watchdog group said the death toll could rise due to the large number of critically wounded people, adding that tens of prisoners, who were incarcerated by the rebels, were also killed as the prison was near the targeted area.Aleppo, Syria's second largest city and once an economic hub, has been carved out between the rebels in the eastern part of the city and the Syrian government forces in the western part.