AMMAN - Syrian President Bashar Assad's forces killed at least 27 people on Wednesday in fighting to fend off a rebel advance on a Syrian military air base near the Lebanese border, opposition campaigners said.Rebel fighters breached defenses of the Qusair Air Defense Base, located some 14 km (nine miles) from Lebanon's Bekaa Valley, a stronghold of the pro-Assad Lebanese Shi'ite movement Hezbollah, after a one-month siege, activists in the area said.But the army countered with rockets and aerial bombings on rebel-held areas in the vicinity, the source said.