Monitoring services have reported that internet access to Syria appears to have been cut off and Google says traffic has dropped off.
US internet security firm OpenDNS said Syria was effectively disconnected from internet communication with the rest of the world late on Monday.
The reports have sparked speculation that the apparent blackout was an attempt by the Syrian government to disrupt the online activities of opponents, according to the Guardian.
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Activists seeking to overthrow president Bashar al-Assad routinely upload videos to YouTube showing atrocities allegedly carried out on behalf of the government.
They also use internet telephony services like Skype to report attacks by government attacks on rebel positions and civilians.
Monitoring companies reported a similar blackout last November, but the Syrian government blamed "terrorists" for the incident.
The Egyptian government of Hosni Mubarak cut the country's internet connection with the rest of the world in January 2011 as protests grew ahead of the government’s ouster.
Last May, security researchers reported that spyware was targeting web users in Syria who were attempting to get around censorship controls using popular proxy tool Simurgh.
The tool allows access to sites blocked by authorities and hides the identity of web users, but some versions of the software have been compromised.
Simurgh is a stand-alone proxy software for Microsoft Windows used mainly by Iranians to bypass censorship since 2009.
But the software is now being recommended and circulated among Syrian internet users for bypassing local censorship.
Researchers at the University of Toronto found that some installation software for Simurgh was also installing keylogging spyware that sends data to a site registered in Saudi Arabia.