President Bashar al- Assad's regime and opposition activists blamed each other for the blackout, which is the first to hit the whole country since Syria's 20-month-old uprising began.
Syrian authorities previously have cut Internet and telephones in areas ahead of military operations. On Friday, some land lines were working sporadically.
The Britain-based Syrian Observatory for Human Rights said the main road to Damascus' airport reopened early Friday afternoon. There were intense clashes after midnight in villages and towns near the facility but the area was calm by the late morning, the group said. It said rebels were able to destroy several army vehicles near the airport.
The Observatory, which has a network of activists around Syria, reported fighting in other southern neighborhoods of Damascus, including Qaboun and Hajar Aswad. The Observatory said it was able to contact its sources who used satellite telephones.
Activists say Assad's regime pulled the plug on the Internet on Thursday, perhaps in preparation for a major offensive. Cellphone service also went out in Damascus and parts of central Syria, they said. The government blamed rebel fighters for the outages.
Thursday's violence appeared to be focused on southern suburbs near the airport, forcing the military to shut the road to the facility. The surrounding districts have been strongholds of rebel support since the uprising began.
Thursday's fighting wounded two Austrian soldiers assigned to the United Nations Disengagement Observer Force in the Golan Heights when their convoy came under fire on the way to the airport, Austria Press Agency said.
The two were transferred to Israel for treatment Friday and their condition is not life-threatening, said David Ratner, a spokesman for Rambam Hospital in Haifa. He said the two soldiers suffered gunshot wounds - one to the chest and the other to the hand.
With pressure building against the regime on several fronts and government forces on their heels in the battle for the northern commercial hub of Aleppo, rebels have recently begun pushing back into Damascus after largely being driven out of the capital following a July offensive. One Damascus resident reported seeing rebel forces near a suburb of the city previously deemed to be safe from fighting.
The Internet outage, confirmed by two U.S.-based companies that monitor online connectivity, is unprecedented in Syria's uprising against Assad, which activists say has killed more than 40,000 people since the revolt began in March 2011.
Regime forces have suffered a string of tactical defeats in recent weeks, losing air bases and other strategic facilities. The government may be trying to blunt additional rebel offensives by hampering communications.
Civilians survival situation worsens
As the rebels and government vie for the upper hand in an increasingly bloody struggle, the conflict's toll on civilians is worsening.
The U.N. refugee agency said Friday it found desperate conditions in the Syrian city of Homs, where thousands of people are living in unheated shelters and a quarter of million people are displaced from their homes.
An assessment team visiting this week saw half of the city's hospitals shut down and "severe shortages of basic supplies ranging from medicine to blankets, winter clothes and children's shoes," the agency's spokeswoman, Melissa Fleming, said.
The violence on the ground, meanwhile, has overshadowed a slow diplomatic process.
Lakhdar Brahimi, the United Nations envoy for Syria, said Thursday that divisions in the Security Council are blocking progress toward ending the violence in Syria, and any eventual cease-fire will require the presence of an international peacekeeping force.
Brahimi said he has the elements for a possible peace plan, but those elements "cannot be put together until the (Security) Council has come together and is ready to adopt a resolution that will be the basis for a political process."
World powers remain divided on how to stop Syria's crisis, with the U.S. and many Arab and European nations calling for Assad to step down, while Russia, China and Iran continue to back the regime. Moscow and Beijing have vetoed three Western-backed Security Council resolutions that would pressure Assad, including with the threat of sanctions, to halt the violence, and the U.N.'s most powerful body remains paralyzed.