BEIRUT — Syrian troops backed by warplanes battled rebels for control of a key highway in Damascus on Saturday, a day after opposition forces cut the strategic artery as part of what they say are efforts to lay the groundwork for an eventual assault on the heavily defended capital.

Rebels have been on the offensive in Damascus since launching a series of attacks on government positions on Wednesday. They brought their fight to within a mile of the heart of the capital on Friday, seizing army checkpoints and cutting a key highway as they pressed their campaign for the city, the seat of President Ba­shar Assad’s power.