Updated

The battle for Syria's second-largest airport intensified on 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 troops 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. Rebels have been trying to capture the international airport for months.

Rami Abdul-Rahman, director of the Britain-based activist group Syrian Observatory for Human Rights, said the current fighting was focused on a section of a highway linking 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 complex in recent weeks, including capturing two army bases along the road to the airport.

The rebels also control large swaths of land outside Aleppo and whole neighborhoods inside the city, which is divided between areas controlled by the regime and others held by rebels. On Saturday, the army launched an offensive on opposition strongholds outside Damascus in an effort to dislodge rebels from areas around the capital that they have been trying to storm for weeks.

On Friday, regime forces fired three missiles into a rebel-held area in eastern Aleppo, hitting several buildings and killing 29 people, according to the Observatory. The group initially reported 14 casualties in the strike that apparently involved ground-to-ground missiles. Some bodies were recovered from the rubble of damaged buildings.

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

A suicide car bombing on Thursday near the ruling Baath Party headquarters in the heart of Damascus killed 53 civilians 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 claimed responsibility for the attack. Car bombs and suicide attacks have been a hallmark of Jabhat al-Nusra, an Islamic militant group fighting among the rebels. 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 neighborhoods.

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 and the Arab League proposed on Wednesday to broker talks between the Syrian opposition and Assad's regime in Moscow, one of Damascus' closest allies. 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 Arab League were trying to establish direct contact between the Syrian regime and the opposition. The Western-backed opposition umbrella group, the Syrian National Coalition, has rejected any talks as long as Assad remains in power.

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

The Coalition said in a statement posted on its Facebook page late Friday that its leaders would not travel to Washington or Moscow for any talks. The coalition 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 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.