Internet services are down across Syria and mobile phone services have been disrupted in some areas, as rebels and government troops clashed near the capital's airport.
The length of Thursday's internet blackout, confirmed by US-based companies that monitor online connectivity, is unprecedented in Syria's 20-month-old uprising against President Bashar al-Assad.
Renesys, a US-based network security firm that studies internet disruptions, said Syria effectively disappeared from the internet at 12:26pm local time.
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It said the main autonomous system responsible for the internet in the country is the Syrian Telecommunications Establishment, and that "all of their customer networks are currently unreachable".
Syrian state TV denied the blackout was nationwide. It said the outage was caused by a technical failure, only affected some provinces, and that technicians were trying to fix the problem.
The government has previously cut phone lines and internet access in areas where regime forces are about to conduct major military operations.
Syria's minister of information said "terrorists", not the state, were responsible for the outage, a pro-government TV station reported.
"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.
In parts of Damascus, mobile and land telephone lines were only working intermittently, as violence raged.
Fighting along the main road to Damascus International Airport, south of the city, temporarily forced the closure of the road.
Dubai-based Emirates airline and EgyptAir have cancelled flights to the Syrian capital due to the fighting.
State media later reported that the road had been "secured" after military intervention in the area.
"The road from the airport was secured after attacks by armed terrorist groups against cars and after a deployment of the competent forces," state television quoted the the information ministry as saying.
Elsewhere in the capital, warplanes bombed the neighbourhoods of Kafr Souseh and Daraya, opposition activists said.
Meanwhile, the joint UN-Arab League special envoy said the old Syria ruled by Assad's family is finished and the "new Syria" will never be the same, in a strong hint that Assad will have to step down before a civil war can end.
Speaking to reporters after briefing the UN Security Council on what he said was the deteriorating situation in Syria, Lakhdar Brahimi did not mention Assad by name.
However, when asked whether a peace plan being considered by diplomats would require regime change, the envoy said: "I think it's very, very, very clear that the people of Syria want change, and real change, not cosmetic changes.
"The new Syria will not look like the Syria of today."
In an apparent reference to the chaotic wartime collapses of the long-entrenched regimes in Libya and Iraq, Brahimi stressed the importance of not allowing state institutions to "wither away".
Brahimi said Syria "very, very urgently" needed a ceasefire and a large peacekeeping force.
Rebels who control large swathes of territory in northern Syria have made military gains in past days, including for the first time shooting down one of the regime's attack aircraft using a surface-to-air missile.
Several rebel brigades attacked the Wadi Deif base in Idlib province on Thursday, opposition sources said, while the army responded with shelling.
Wadi Deif is the main base for government forces in the area and the location of their main fuel depot.
Fighting was also reported outside the nearby rebel-held town of Maaret al-Numan.
In the country's commercial capital, Aleppo, an air strike on Thursday killed at least 15 civilians, according to the Syrian Observatory for Human Rights.
"At least 15 people, among them five children and two women, were killed when a warplane dropped two bombs on the Ansari district of Aleppo," the group's head Rami Abdelrahman told the AFP news agency.
The opposition says more than 35,000 people have been killed since the uprising began in March last year. Hundreds of thousands of people have fled to neighbouring countries.