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 said he saw black smoke rising from the east and the south of the city on Friday morning and could hear the constant boom of shelling. State television said Assad's forces were fighting rebels in those areas.

An aviation source in neighbouring Jordan said two Syrian Air flights crossed Jordanian air space heading for the Syrian capital on Friday evening and that Damascus airport was open, although international airlines were staying away.

The head of the national airline Syria Air said services were operating according to schedule, state television reported.

EgyptAir and Emirates have suspended flights to Damascus in response to the recent violence and there was no sign that Air Arabia and flydubai had flown scheduled trips on Friday.

"Airlines are not operating to Damascus today," said a Dubai-based airline official.

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.

A rebel contacted by Reuters who said he was on the airport road said his fighters would not let the airport operate. "We will never open the road. It's still closed and it will remain closed. We will not allow planes to arrive," he said.

MORTAR FIRED

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.

U.S. 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.

Global hacking network Anonymous said it would shut down Syrian government websites around the world in response to a move it said was aimed at silencing Assad's critics.

"As we discovered in Egypt, where the dictator (Hosni) Mubarak did something similar - this is not damage that can be easily or quickly repaired," it said, referring to an Internet outage during the early days of the 2011 uprising in Egypt.

French foreign ministry spokesman Philippe Lalliot said the communications cut was of a matter of "extreme concern".

"It is another demonstration of what the Damascus regime is doing to hold its people hostage. We call on the Damascus regime to reestablish communications without delay," he said.

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.

"That is unlikely to have happened," it said. (Additional reporting by Oliver Holmes in Beirut, Suleiman al-Khalidi in Amman, Mariam Karouny, Praveen Menon in Dubai and Jim Finkle in Boston; editing by Anna Willard)

• A Permanent Economic Emergency