Opposition activists and monitoring firms say Syria has cut internet and phone services for the first time since the civil war started more than 18 months ago.
It is believed the government is trying to thwart rebel communications after suffering a series of setbacks in fighting recently.
There are reports rebel fighters have moved to within three kilometres of the Damascus international airport, and there are numerous reports of mortar fire nearby.
Monitoring companies said Syria cut all its links to the internet, and it is not clear if service has been restored.
Akamai, a firm which monitors global traffic, said traffic stopped from 10:26am GMT on Thursday, supporting the observation from another IT firm, Renesys, "that Syria is effectively off the internet."
Renesys said in a blog posting that its monitoring showed "Syria's international internet connectivity shut down".
"Looking closely at the continuing internet blackout in Syria, we can see that trace routes into Syria are failing, exactly as one would expect for a major outage," Renesys chief technology officer James Cowie said.
Mr Cowie said in a blog posting that "there are a few Syrian networks that are still connected to the internet... but the originator of the routes is actually Tata Communications. These are potentially offshore, rather than domestic, and perhaps not subject to whatever kill switch was thrown today within Syria."
According to activists, sudden communication cuts regularly occur before major military offensives.
Amnesty International said on Twitter that the reports of the internet shutdown were "very disturbing."
In early 2011, Egypt's then-president Hosni Mubarak cut his 80 million people off from the web before his regime was toppled by activists.
The shutdown in Egypt was the most comprehensive official electronic blackout of its kind, experts said at the time.
As fighting raged near the airport, Egypt Air and Emirates both suspended services in and out of Syria.
Government forces are reportedly moving to try to keep access roads to the airport from falling under rebel control.
The loss of the international airport would be a symbolic blow for the regime, but not a fatal one.
There are several other military airfields in Damascus still in government hands.