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 2-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.

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 on 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 earlier this week, marked the most sustained challenge to the heart of Damascus, the seat of Assad’s power.  

A suicide car bombing on Thursday near the ruling Baath Party headquarters in central Damascus killed 53 people and wounded more than 200, according to state media. Anti-regime activists put the death toll at 61, which would make it the deadliest bombing of the revolt in the capital.

The different tolls could not be reconciled.

Nobody has claimed responsibility for the attack. Car bombs and suicide attacks have been a hallmark of Jabhat al-Nusra, an Islamic militant group that is one of the myriad factions fighting on the rebel side. Nusra fighters, the most effective group on the battlefield, have led assaults on military installations and control swaths of territory in the north, including parts of Aleppo.

The fighting has increasingly taken on sectarian overtones with members of Syria’s Sunni Muslim majority dominating the rebel ranks, who are fighting Assad’s regime that is mostly made up of Alawites, an offshoot Shiite group.  

Efforts to stop the bloodshed in Syria so far have failed, leaving the international community at a loss of how to end the civil war.

Russia, one of Assad’s closest allies, and the Arab League proposed on Wednesday to broker talks between the Syrian opposition and Assad’s regime. Syrian Foreign Minister Walid al-Moallem will lead a delegation to Moscow on Monday, and Russia had been expecting a visit in March from opposition leader Mouaz al-Khatib.

Russian Foreign Minister Sergey Lavrov said the Kremlin and the League were trying to establish direct contact between the Syrian regime and the opposition. The Western-backed opposition umbrella group, the Syrian National Coalition, long rejected any talks as long as Assad remains in power.

In a sharp turnaround, al-Khatib said earlier this month he would meet with members of the regime if that would help end the bloodshed. His comments, however, drew pointed criticism from several opposition figures who said al-Khatib spoke for himself, not the group.

On Friday, the Coalition announced after two days of meetings in Cairo that it would welcome U.S. and Russian mediation to negotiate a peace deal to end the country’s civil war but insisted it would not allow Assad or members of his security services to participate in the talks.

But the SNC then said in a statement posted on its Facebook page late Friday that its leaders would not travel to Washington or Moscow for any talks. It said the decision was taken to protest the international community’s “silence over crimes committed by the regime” against Syrian people in Aleppo and other cities across the country.

The Coalition also lashed out at Russia, saying it bears “special responsibility” because it supplies the regime with weapons.

The statement also said that the opposition leaders would boycott a meeting next month in Rome of the Friends of Syria, which includes the United States and its European allies.