Syrian air force jets bombarded rebel targets close to the Damascus airport road and a regional airline said the violence had halted international flights to the capital.
Activists said security forces clashed with rebels trying to topple President Bashar al-Assad around Aqraba and Babilla districts on the southeastern outskirts of the Damascus which lead to the international airport.
Internet connections and most telephone lines were down for a second day, the worst communications outage in a 20-month-old uprising in which 40,000 people have been killed, hundreds of thousands have fled the country, and millions been displaced.
The mostly Sunni Muslim rebels who are battling Assad, from Syria's Alawite minority linked to Shi'ite Islam, have been making gains around Syria by overrunning military bases and have been ramping up attacks on Damascus, his seat of power.
A resident of central Damascus told Reuters he could see black smoke rising from the east and the south of the city on Friday morning and could hear the constant boom of shelling.
"Airlines are not operating to Damascus today," said a Dubai-based airline official. EgyptAir and Emirates suspended flights to Syria on Thursday.
The Syrian Observatory for Human Rights, a British-based opposition monitoring group, said jets were bombarding targets in rural areas around Aqraba and Babilla, where rebels clashed with Assad's forces.
The Observatory's director, Rami Abdelrahman, said the airport road was open, but there was minimal traffic.
Syrian authorities said late on Thursday that the airport road was safe after security forces cleared it of 'terrorists' - the label Damascus uses to describe Assad's armed opponents.
Rebels said that at least one mortar round was fired at the airport during clashes on Thursday.
"We want to liberate the airport because of reports we see and our own information we have that shows civilian airplanes are being flown in here with weapons for the regime. It is our right to stop this," rebel spokesman Musaab Abu Qitada said.
US and European officials said rebels were making gains in Syria, gradually eroding Assad's power, but said the fighting had not yet shifted completely in their favour.
A Damascus-based diplomat said he believed the escalation in fighting around the capital was part of a government offensive which aimed to seal off the state-controlled centre of the city from rebel-held rural areas to the south and east.
Activists say Assad's forces have also been shelling the Daraya district to the southwest of the city, trying to prevent rebels from cementing their hold of an area which could give them a presence in a continuous arc from the northeast to southwest of the capital's outer districts.
"I don't know whether the shelling has succeeded in pushing back the FSA (rebels) - experience shows that they return very quickly anyway," the diplomat said. "We seem to be entering a decisive phase of the Damascus offensive."
Syria's Internet shut down on Thursday, a move which activists blamed on authorities but which authorities variously attributed to a 'terrorist' attack or a technical fault.
CloudFlare, a firm that helps accelerate Internet traffic, said on its blog that saboteurs would have had to simultaneously cut three undersea cables into the Mediterranean city of Tartous and also an overland cable through Turkey in order to cut off the entire country's Internet access.