Syria has been cut off from the Internet, technology companies which monitor web traffic reported, as the US government blamed the embattled regime of Bashar al-Assad for the move.
Akamai, a firm which monitors global traffic, said traffic stopped from 1026 GMT, and that this supports 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."
Renesys chief technology officer James Cowie said: "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."
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."
He added that the functioning networks "were implicated in the delivery of malware targeting Syrian activists in May of this year."
The French-based monitoring firm Cedexis also reported that traffic was down, and a spokesman told AFP that "it appears to be a voluntary cut" of IP traffic.
US officials blamed Assad for the shutdown as he cracks down on rebels in the war-wracked country.
"Obviously, we condemn this latest assault on the Syrian people's ability to express themselves and communicate with each other and it just, again, speaks to the kind of desperation of the regime as it tries to cling to power," State Department spokeswoman Victoria Nuland said.
Nuland added, however, that some 2,000 communications sets supplied to the opposition rebels over the past months as part of a US non-lethal assistance program were not affected by the blackout.
"They are all designed to be independent from and able to circumvent the Syrian domestic network, precisely for the reason of keeping them safe, keeping them secure from regime tampering, regime listening, regime interruption," she said.
According to activists, sudden communication cuts regularly occur before major military offensives.
Earlier, activists in Syria said Internet and mobile telephone communications were cut in the capital Damascus, and land lines were barely functioning.
An activist in the embattled Eastern Ghouta region east of the capital told AFP that only satellite Internet connections were operating in the area.
The Local Coordination Committees, a leading network of activists on the ground, said "communications and Internet service have been cut in most parts of Damascus and its suburbs, raising fears that the criminal Syrian regime is up to something."
It said landline and mobile services were cut throughout the central provinces of Homs and Hama, in Daraa and Suweida provinces in the south, in Tartus province on the coast, and in some cities in Deir Ezzor and Raqa provinces in the east.
Official news agency SANA also saw its feed interrupted at midday.
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.
Egypt's four main Internet service providers cut off international access to their customers in a near simultaneous move two days after anti-Mubarak protests began.
The shutdown in Egypt was the most comprehensive official electronic blackout of its kind, experts said at the time.