Internet and cut mobile phone services across Syria have been cut as rebels and government troops waged fierce battles near the capital's airport.
International airlines were forced to suspend flights.
The internet blackout, confirmed by two US-based companies that monitor online connectivity, is unprecedented in Syria's 20-month-old uprising against President Bashar Assad.
Regime forces have suffered a string of tactical defeats in recent weeks - losing air bases and other strategic facilities - and the blackout may be an attempt by the government to blunt any further rebel offensives by hampering communications.
Authorities often cut phone lines and internet access in select areas where regime forces are conducting major military operations to disrupt rebel communications. Activists in Syria reached by satellite telephone confirmed the blackout.
Syrian state TV denied the blackout was nationwide and said the outage was caused by a technical failure that only affected some provinces.
Syria's information minister said "terrorists" had cut off the internet and that engineers were working to repair the fault.
"It is not true that the state cut the internet. The terrorists targeted the internet lines, resulting in some regions being cut off," he was quoted by al-Ikhbariya as saying.
Renesys, a US-based network security firm that studies internet disruptions, said Syria effectively disappeared from the internet at 12:26 pm local time.
Akamai Technologies Inc also confirmed a complete shutdown for Syria.
With pressure building against the regime on several fronts, rebels have been trying to push their way back into the capital after being largely driven out after a July offensive into Damascus.
Opposition fighters were battling government troops near the city's international airport yesterday, the Britain-based Syrian Observatory for Human Rights said, prompting the military to close the road to the facility.
The airport lies on the capital's southern outskirts, and the surrounding districts have been strongholds of support for the rebels since the start of the uprising.
Government warplanes struck the rebellious districts around Damascus today, including Daraya, where fighting has raged for days, as rebels fight their way into the capital, the Observatory said.
In the country's south, rebels bombed the house of a top member of the country's ruling Baath party, killing him and his three body guards, activists said.