The internet has been cut off and mobile phones have been disrupted in Syria, monitoring firms have said.
Networking firm Renesys said the country's connection protocols were unreachable, "effectively removing the country from the internet".
Local reports suggested that the internet had been down since early afternoon, and that telephone lines were only working intermittently.
The Syrian government has blamed "terrorists" for the disconnection.
"The terrorists targeted the internet lines, resulting in some regions being cut off," Syria's minister of information told a pro-government television station.
According to activists, it has been known for similar communication cuts to occur in isolated areas before military operations.
Amnesty International has described the reports as "very disturbing".
Renesys, a US-based company which tracks internet connectivity worldwide, said on its blog: "In the global routing table, all 84 of Syria's IP address blocks have become unreachable."
According to its systems, access was disconnected at 12:26pm local time (10:26 GMT).
'Started to dive'
Another US firm, Akamai, also confirmed it was unable to connect with Syria's internet.
Activists were using satellite phones to make contact with the outside world, Reuters reported.
Search giant Google noted that Syrians were unable to access any of its services including YouTube. The video sharing website is a popular place for activists to upload footage from the country.
Psiphon, a Canadian company that produces advanced computer systems for circumventing censorship systems, told the BBC that its monitoring showed the number of people connecting from within Syria had "started to dive" from around midday local time.
Psiphon's system - which requires specialised software - has throughout November been seeing 13-15,000 log-ins per day.
However, at the time of the reported outage, none of its users were able to get online.
Syria has previously seen large outages in July and August this year, each lasting less than an hour and only affecting targeted areas.
During the uprising in Egypt, four major internet service providers were cut off in the country during mass protests against the then-President Hosni Mubarak.
Egyptians quickly found ways around the blocks, and Google introduced a "speak-to-tweet" service which allowed people to connect to Twitter via the telephone.
In Libya, internet blackouts were common in areas that were at the time still controlled by Colonel Gaddafi.
The exact method being used to cut off the internet in Syria is unknown, one security expert said, but there are clues.
"It looks like they are using the same approach as Libya did," explained Rik Ferguson, vice president of security research at Trend Micro.
"Requests for Syrian addresses are simply timing out - so it's likely to be 'blackholing' or even breaking connections physically by cutting cables or switching things off."
Blackholing is a tactic which involves sending internet traffic into a dead end - rather than its intended destination.