Syria was cut off from the Internet on Tuesday, according to US tech firms monitoring Web traffic and the State Department.
The reasons were not immediately clear, but a similar blackout happened last November.
"Syria is currently experiencing an internet blackout as of this afternoon," a State Department tweet said.
Umbrella Security Labs reported a "significant drop in traffic from Syria" starting around 1845 GMT.
"On closer inspection it seems Syria has largely disappeared from the Internet," said Umbrella's Dan Hubbard.
A similar picture came from Google through its Transparency Report on Web traffic and Web monitoring firms Akamai and Renesys.
According to activists, sudden communication cuts may occur before major military offensives.