BEIRUT — Dozens of families and some opposition fighters started leaving besieged rebel-held neighborhoods in the northern Syrian city of Aleppo on Saturday after the government opened safe corridors for civilians and fighters who want to leave, state media reported. The government completely closed the main road into rebel-held areas of Aleppo on July 17, effectively besieging the 300,000 people living there. Earlier this week, Syrian President Bashar Assad offered an amnesty to rebels who lay down their arms and surrender to authorities in the next three months. Opposition activists denied reports that Aleppo residents were leaving rebel-held neighborhoods of the city, saying that state media was attempting to falsely suggest that civilians were fleeing the area in large numbers.