More than 60,000 people have been killed in the 21-month-old uprising and civil war, the UN said this week, sharply raising the death toll in a conflict that shows no sign of ending.
After dramatic advances over the second half of 2012, the rebels now hold wide swathes of territory in the north and east, but are limited in exerting control because they cannot protect towns and villages from Mr Assad's helicopters and jets.
Hundreds of fighters from rebel groups were attempting to storm the Taftanaz air base, near the northern highway that links Syria's two main cities, Aleppo and the capital Damascus.
Rebels have been besieging air bases across the north, in the hope this will reduce the Government's power to carry out air strikes and resupply loyalist-held areas.
Rami Abdulrahman, head of the opposition-aligned Syrian Observatory for Human Rights which monitors the conflict from Britain, said as many as 800 fighters were involved.