More than 90% of the Internet access in Syria was shut down on Thursday, according to the Internet monitoring group Renesys.
It was not clear who was behind the latest event, but the government has intermittently cut off Internet access several times in the past two years.
Opposition activists often transmit updates about the civil war in reports and images on the Web.
Syria state TV reports that the government's minister of communications said maintenance workshops are working on "fixing the blackout in the main communication and Internet network in a number of Syrian provinces."
Syria has shut down the Internet in the past, said U.S. Ambassador to Syria Robert Ford.
He added that the United States has given "a thousand pieces of non-lethal equipment -- largely communications gear" to help opposition activists get around blocks to the Internet.
He was speaking in Washington on Wednesday about the humanitarian situation in the country. He talked to CNN Thursday.
"The Syrian government has been monitoring (the Internet) for years. They have been using the Internet with Iranian assistance to track opposition activists, arrest and kill them," Ford said.
"That is the reason why our non-lethal assistance to the Syrian opposition, we put a special emphasis on communications equipment precisely to help the Syrian people tell the world what is going on inside Syria," he said.
"A lot of the pictures that you see on the nightly news are from communication equipment that we supply to very brave and very dedicated opposition activists inside Syria," Ford said. "We have provided over a thousand pieces of non-lethal equipment -- largely communications gear to help them get around the restrictions on the Internet that the Syrian government imposes."
Meanwhile, the road to Damascus International Airport has been shut down because of continuing clashes and military operations in towns on the outskirts of the city, the London-based Syrian Observatory for Human Rights said.
Egypt Air is canceling flights to Syria starting Friday until further notice, said Egypt Air spokesman Mohamed Rahma.
The airline cited the "deteriorating situation" around the Damascus airport, a Cairo airport official said, according to Egyptian semi-official news agency Al-Ahram.
Syrian regime forces are fighting rebels as they attempt to secure the road to the Damascus airport and surrounding areas, the London-based opposition group, Syrian Observatory for Human Rights, said Thursday. The fighting has gotten close to the airport and Syrian regime forces want to get those areas under control, the group said.
These events are part, some believe, of a possible turning point in the nearly two-year war.
On Wednesday villagers in northern Syria picked pieces of a downed fighter jet from an olive grove after rebel fighters claimed to have shot down three government aircraft in 24 hours.
Syrian President Bashar al-Assad's government has relied more on air power to battle the 21-month-old revolt against it, and witnesses said a cheer went up when the jet went down near Aleppo.
"We want to take this ... to show them in the other villages," a man who identified himself as Abu Dargham told CNN as he showed off two twisted chunks of metal. "Let them see what happened to these planes."
The downed plane's tail was largely intact, but the fuselage was in pieces and the type of aircraft was not immediately identifiable. Locals picked it apart, some of them stuffing pieces into bags as a tractor hauled away what appeared to be an engine. Cheering children were piled on the tractor as it drove away.
Witnesses said two fliers ejected from the plane before the crash. One was found unconscious and taken to a makeshift clinic, while villagers said they were still searching for the other late Wednesday.