Syrian rebels battled forces loyal to President Bashar al-Assad just outside Damascus on Thursday, forcing the closure of the main airport road, and the Dubai-based Emirates airline suspended flights to the Syrian capital.
Residents also reported Internet connections in the capital were down and mobile and land telephone lines working only sporadically in what appeared to be the worst disruption to communications in Syria since an uprising began 20 months ago.
The past two weeks have seen rebels overrunning army bases across Syria, exposing Assad's loss of control in northern and eastern regions despite the devastating air power that he has used to bombard opposition strongholds.
Rebels and activists said the fighting along the road to Damascus airport, southeast of the capital, was heavier in that area than at any other time in the conflict.
The Syrian Observatory for Human Rights, a opposition monitoring group, said clashes were particularly intense in Babbila, a suburb bordering the insurgent stronghold of Tadamon.
Nabeel al-Ameer, a spokesman for the rebel Military Council in Damascus, said that a large number of army reinforcements had arrived along the road after three days of scattered clashes ending with rebels seizing side streets to the north of it.
"There are no clashes directly around the airport; the fighting is about 3 or 4 kilometres away," he said via Skype, adding that rebels had taken control of many secondary roads and were expected to advance towards the airport.
He said that he hoped the proximity of the rebels to the airport would dissuade authorities from using it to import military equipment, but the priority now was to block the road.
A Syrian security source told Reuters on condition of anonymity that the army had started a "cleansing operation" in the capital to confront rebel advances.
Residents said the Internet in Damascus crashed in the early afternoon and mobile and land telephone lines were functioning only intermittently.
A blog post on Renesys, a U.S. company which tracks Internet traffic worldwide, said that at 12:26 p.m. in Damascus, Syria's international Internet connectivity shut down completely.
Emirates said it was suspending daily flights to Damascus "until further notice", but other airlines continued operations.
Airport sources in Cairo said an Egypt Air flight that left at 1:30 p.m. (1130 GMT) had landed in Damascus as scheduled.
"The Egypt Air plane has arrived ... and passengers are all safe but the pilot was instructed to take off back to Cairo without passengers if he felt that the situation there is not good to stay for longer," an official at Cairo airport said.
Elsewhere in Damascus, warplanes bombed Kafr Souseh and Daraya, two neighbourhoods that fringe the centre of the city where rebels have managed to hide out and ambush army units, according to opposition activists.
"Not Last Days Yet"
A senior European Union official said that Assad appeared to be preparing for a military showdown around Damascus, possibly by isolating the city with a network of checkpoints.
"The rebels are gaining ground but it is still rather slow. We are not witnessing the last days yet," the official said on condition of anonymity.
"On the outskirts of Damascus, there are mortars and more attacks. The regime is thinking of protecting itself ... with checkpoints in the next few days ... (It) seems the regime is preparing for major battle on Damascus."
In the north of the country, rebel units launched an offensive to seize an army base close to the main north-south highway that would allow them to block troop movements and cut Assad's main supply route to Aleppo, Syria's biggest city.
The Observatory said that rebel units from around Idlib province massed early on Thursday morning to attack Wadi al-Deif, a base east of the rebel-held town of Maarat al-Numan.
Wadi al-Deif has been a thorn in the side of rebel units who first besieged the station in October but met fierce resistance from government forces, backed up by air strikes.
Assad is fighting an insurgency that grew out of peaceful demonstrations for democratic reform but escalated, after a military crackdown on protesters, into a civil war in which 40,000 people have been killed.
Most foreign powers have condemned Assad but stopped short of arming rebel fighters as they fear heavy weapons could make their way into the hands of radical Islamist units, who have grown increasingly prominent in the insurgency.
Rebels decry their supporters for not providing them with surface-to-air missiles that they say they need to counter the air force. But recent looting of anti-aircraft missiles from army bases has allowed them to shoot down helicopters and jets.
"So far, there is no evidence that any of the surface-to-air missiles used to date have come from outside Syria," said Peter Bouckaert, emergencies director at Human Rights Watch.
"The limited number of surface-to-air missiles that have shown up all appear to have come from Syrian military stock captured by the armed opposition."
He said the number of these missiles in rebel hands was probably over 20 but that will rise significantly as rebels are capturing military bases on an almost-daily basis.
The relatively small number of anti-aircraft missiles looted so far means that many rebel-controlled areas of the country remain vulnerable to air strikes. The Observatory said 15 citizens, including children and women, were killed during a bombing in Aleppo's Ansari district on Thursday.
Activist video footage showed the bodies of at least four children, wrapped in red blankets and apparently wearing pyjamas. Another video showed the immediate aftermath of the attack, with the bodies of children in the street and covered in cement dust. Half of one young boy's head was missing.