There are reports the main road to the airport of the Syrian capital Damascus has been closed and some flights have been suspended amid fierce fighting in the area.
Rebels say there were clashes all along the road and mortars have been fired at the airport runway.
Internet and mobile phone systems also went down on Thursday. Land lines were functioning only intermittently.
It appears to be the worst disruption to communications since the uprising began 20 months ago.
The government has previously cut access to the internet during major operations, but analysts say a nationwide switch-off is unprecedented.
