Two US-based internet-monitoring companies say Syria has shut off the internet nationwide.
A blog post on Renesys, a US company which tracks internet traffic worldwide, said that at 12.26pm in Damascus, Syria's international internet connectivity shut down completely.
Akamai Technologies also confirmed a complete outage.
The Syrian government has previously cut phone lines and internet access in areas where regime forces were conducting major military operations.
An activist near Damascus who gave his name as Abu Sham said the government had cut the internet in the southern neighbourhoods of the capital on Thursday.
Another activist, Abu Qais al-Shami, who lives outside the country, also said land-lines, mobile phone signals and the internet were cut in several of the capital's southern neighbourhoods, including Yarmouk and Tadamon, around noon.
He said he had been able to communicate with contacts in Syria by using satellite telephones.
Elsewhere in the capital, warplanes bombed Kafr Souseh and Daraya, two neighbourhoods that fringe the centre of the city where rebels have managed to hide out and ambush army units, opposition activists said.
The past two weeks have seen military gains by rebels who have stormed and taken army bases across Syria, exposing Assad's loss of control in northern and eastern regions despite the devastating air power he has used to bombard opposition strongholds.
According to a pro-government TV station, Syria's minister of information said that "terrorists", not the state, were responsible for the outage.
"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.
State TV quoted the telecommunications minister as saying that engineers were working to repair what he said was a fault.
Meanwhile Syrian rebels battled forces loyal to President Bashar al-Assad just outside Damascus on Thursday, restricting access to its international airport. The Dubai-based Emirates airline suspended flights to the Syrian capital.
A rebel fighter who identified himself as Abu Omar, a member of the Jund Allah brigade, told Reuters that insurgents fired mortars at the airport's runways and were blocking the road linking it with the capital.
He said insurgents were not inside the airport but were able to block access to and from it. Another source in a Damascus rebel unit said mortars had been used in clashes near the airport but did not know whether rebels had fired mortars directly at the airport.
Their accounts could not be immediately verified because of severe restrictions on media access to Syria.
An official at EgyptAir said it had cancelled its Friday flight to Damascus due to the "deteriorating situation" around the airport. He said the airline would hold an urgent meeting in the next few hours with Egyptian officials to discuss halting all flights between Egypt and Syria.
