Jim Cowie, the chief technology officer at Renesys, said the abruptness of the outage suggested it wasn't due to a severed cable. Syria has several cables that connect it to the outside world, and all of them would have had to be cut at once for a complete outage. A power outage or an intentional shutdown at central Syrian telecommunications facilities is a more likely cause, he said.
"We saw everything go in three to four minutes, which looks like a light switch," Cowie said.
He said the profile of the outage was similar to what the Egyptian government did in January 2011 during the Arab Spring uprising that toppled longtime leader Hosni Mubarak. Egypt switched off the Internet for five days, halting businesses, banking and — at the height of the demonstrations — the ability of protest leaders to organize and communicate with each other.
Bahrain's Sunni rulers also jammed cellphones during the military offensive on the protesters' encampment in the capital of Manama in March 2011. Internet service remained at a crawl when the Bahrain's military stormed the city's Pearl Square — the headquarters of the revolt — after weeks of street protests.
Ann Harrison, deputy program director for the Middle East and North Africa at Amnesty International, said in a statement that the group worried the communications were cut in Syria "to shield the truth of what is happening in the country from the outside world."
The shadowy group of hacker-activists known as Anonymous sent out a tweet Thursday saying that as of 9 p.m. (0200 GMT Friday) it would "begin removing from the Internet all web assets belonging to the Assad regime that are NOT hosted in Syria. We will begin with the websites and servers belonging to ALL Syrian Embassies abroad, which we will begin systematically removing from the Internet tonight." It said the first target was the website of the Syrian Embassy in China.
"By turning off the Internet in Syria, the butcher Assad has shown that the time has come for Anonymous to remove the last vestiges of his evil government from the Internet," Anonymous said in its statement.
Thursday's violence appeared to be focused on southern suburbs near the Damascus international airport, forcing the military to shut the road to the facility. The surrounding districts have been strongholds of rebel support since the uprising began.
At the United Nations, the secretary-general's office said at least four soldiers assigned to the U.N. Disengagement Observer Force in the Golan Heights were injured in the crossfire on the airport road as their unit was heading out for a routine rotation of forces.
Rami Abdul-Rahman, head of the Britain-based Syrian Observatory for Human Rights, said the regime has started a major offensive around the airport where rebels have been particularly active in recent weeks.
Abdul-Rahman, who relies on a network of activists in Syria, said large convoys of government reinforcements were seen heading south toward the airport, which is 25 kilometers (15 miles) southeast of Damascus. The fighting was concentrated in and around the suburbs of Aqraba and Beit Saham, he said.
The Syrian Information Ministry later said the airport road was secure after attacks by "terrorist groups" on motorists, according to state TV. It was not immediately clear whether the road had been reopened.
The fighting prompted both Emirates airline and EgyptAir to cancel flights to Damascus.
Despite months of sporadic fighting and deteriorating security in Damascus, the airport has remained open.
But EgyptAir said in a statement that the airline will halt all flights to Damascus and Aleppo starting Friday, until further notice. EgyptAir head Rushdi Zakaria said the decision was due to deteriorating security conditions in Syria.
Syrian TV also said government forces were chasing "al-Qaida elements" around Damascus, mostly in the eastern suburbs of Douma and the southern suburb of Daraya.