Activists said President Bashar Assad's regime pulled the plug on the Internet, 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.
With pressure building against the regime and government forces on their heels in 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 20-month-old uprising against Assad, which activists say has killed more than 40,000 people.
Regime forces suffered a string of defeats in recent weeks, losing air bases and other strategic facilities. The government may be trying to blunt new rebel offensives by hampering communications.
U.S. State Department spokeswoman Victoria Nuland condemned what she called the regime's "assault" on Syrians' ability to communicate with each other and express themselves. She said the move spoke to a desperate attempt by Assad to cling to power.
Thursday's violence appeared to be focused on southern suburbs near the Damascus international airport, forcing the military to shut the road to the facility. The fighting prompted both Emirates airline and EgyptAir to cancel flights to Damascus.
At the United Nations, the secretary-general's office said at least four soldiers assigned to the U.N. Disengagement Observer Force in the Golan Heights were injured in the crossfire on the airport road.