The firm said in a short statement: "Syria's international Internet connectivity shut down. In the global routing table, all 84 of Syria's IP address blocks have become unreachable, effectively removing the country from the Internet."
Expanding later in the day at 3:45 p.m. UTC, Renesys said that traceroutes into Syria were failing, "exactly as one would expect for a major outage."
"Now, there are a few Syrian networks that are still connected to the Internet, still reachable by traceroutes, and indeed still hosting Syrian content. These are five networks that use Syrian-registered IP space, but the originator of the routes is actually Tata Communication," Renesys chief technology officer Jim Cowie wrote.
He added: "These are potentially offshore, rather than domestic, and perhaps not subject to whatever killswitch was thrown today within Syria."
Pointing to a status page, Renesys noted that 77 networks experienced an outage, representing around 92 percent of the routed networks in the country. The research firm said that all of the networks connected to the outside world through the state-owned Syrian Telecommunications Establishment.
Google's Transparency Report, which monitors Internet connectivity events around the world, also confirmed the outage as widespread across Syria. The search giant's report is updated throughout the day but was not updated until around 4:30 p.m. UTC -- close to six hours after the outage was first noted.
Akamai Technologies also confirmed the reports, describing it as a "complete outage" for the entire country. Tweeting a graph showing rising traffic throughout the morning -- traffic tends to dip locally during the night -- and then suddenly dropping at around 10:26 a.m. UTC.
Amid a major military operation, the BBC says, the Syrian government has previously cut off Internet access to the country before, suggesting this may be yet another government-controlled disconnection. The last major outage in Syria was in August 2011, more than a year ago.
Syria's infromation minister said "terrorists" were the cause of the Internet blackout, not the state, according to pro-government Ikhbariya TV reported via the Reuters news agency.
Many are taking to Twitter to tweet under the #SyriaBlackout hashtag that many cannot communicate with those in the Middle Eastern country, including by Web or by phone calls.
Tweeting my colleague, Charlie Osborne, one apparent member of Anonymous tweeted the following, in order to get those back online using somewhat primitive dial-up connections. (It's worth noting that during both the Egyptian and Libyan uprising, some of the older technologies remained intact, while broadband and mobile communications were cut off.)
During the final days of the falling Gaddafi regime, only a couple of months before he was summarily executed in the street by rebel fighters, Libya's nationwide DSL Internet access flickered back to life for a short while after Libyan opposition forces stormed the capital, Tripoli. Internet access had been cut off in the country for more than six months.
Social media was used during the 2010-2011 Arab Spring, notably in Egypt, Tunisia and Libya in a bid to gather masses of people to central areas to protest at the oppressive, often non-elected governments. But while the Syrian people began to rise up against the dictatorship of President Bashar al-Assad, the people continue nonetheless to try and regain their country as their own.
We will keep this post updated throughout the day with any major developments.
Updated at 4:50 p.m. GMT:with update from Google Transparency Report.
Updated at 5:15 p.m. GMT: with statement from Syrian government; updated Renesys statement.