DAMASCUS, Syria — Syrian elite forces backed by Hezbollah fighters besieged a rebel-held mountain resort near the border with Lebanon on Saturday under the cover of intense air strikes, activists and official media outlets reported. Hezbollah’s Al-Manar TV aired footage from the battlefield showing smoke billowing from much of Zabadani as shells and missiles struck the town. Syrian state TV quoted an unnamed military official as saying that “terrorists suffered large losses.” On Friday, Islamic militants and rebels launched fresh attacks on government-held neighborhoods in Aleppo, setting off some of the heaviest fighting in months in the contested city that was once Syria’s commercial center.