BEIRUT — The battle for Syria’s second-largest airport intensified Saturday as government troops tried to reverse recent strategic gains the rebels have made in the northeast in their quest to topple President Bashar Assad.

Assad’s forces have been locked in a stalemate with rebels in Aleppo since July when the city, the largest in Syria, became a major battlefield in the two-year-old conflict the United Nations says has killed at least 70,000 people. For months, rebels have been trying to capture the international airport, which is closed because of the fighting.

Additional Photos

Smoke rises from heavy shelling in Syria’s largest city, Aleppo, on Tuesday in this citizen journalism image provided by Aleppo Media Center AMC, which has been authenticated based on its contents and other reporting. The Associated Press

Rami Abdul-Rahman, director of the Britain-based Syrian Observatory for Human Rights activist group, said the current fighting was focused on a section of a highway linking the airport with Aleppo, the commercial hub of the nation.

The rebels have cut off the highway, which the army has been using to transport troops and supplies to a military base within the airport complex. Rebels have made other advances in the battle for the airport in recent weeks, including overrunning two army bases along the road to the airport.

The rebels also control large swaths of countryside outside Aleppo and whole neighborhoods inside the city, which is carved up into areas controlled by the regime and others held by rebels. Months of heavy street fighting has left whole neighborhoods of the storied city in ruins.

On Friday, regime forces fired three missiles into a rebel-held area in eastern Aleppo, hitting several buildings and killing 37 people, according to the Observatory. Some bodies were recovered from the rubble of apartments flattened in the strike, which apparently involved ground-to-ground missiles.

A similar attack Tuesday in another impoverished Aleppo neighborhood killed at least 33 people, almost half of them children.

In Damascus, government forces shelled several rebellious suburbs Saturday as part of their efforts to dislodge opposition fighters who have used the towns and villages surrounding the capital as a staging ground for their attempts to push into the center of the city.

Recent rebel advances in the suburbs, combined with the bombings and three straight days of mortar attacks last week, marked the most sustained challenge to the heart of Damascus, the seat of Assad’s power.

A suicide car bombing Thursday near the ruling Baath Party headquarters in central Damascus killed 53 people and wounded more than 200, according to state media.

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