Syrian authorities closed Aleppo airport as well as all roads leading into the city on Saturday, three military sources told Reuters, as opposition forces against President Bashar al-Assad said they had reached the heart of Aleppo.The opposition fighters, led by the opposition militant group Hayat Tahrir al-Sham, carried out a surprise sweep through government-held towns this week and reached Aleppo nearly a decade after having been forced out by al-Assad and his allies.For all the latest headlines follow our Google News channel online or via the app.Russia, one of al-Assad’s key allies, has promised Damascus extra military aid to thwart the opposition, two military sources said, adding new hardware would start arriving in the next 72 hours.The Syrian army has been told to follow “safe withdrawal” orders from the main areas of the city that the opposition forces have entered, three army sources said.The fighters began their incursion on Wednesday and by late Friday an operations room