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.
The Syrian Observatory for Human Rights said that fighting along the road to the airport, southeast of Damascus, was heavier in that area that at any other time in the 20-month-old uprising against Assad.
"As of 20 minutes ago, there was heavy fighting along all the areas along the road," the British-based Observatory's director Rami Abdelrahman told Reuters by telephone. He said clashes were particularly intense in Babbila, a southern suburb bordering the insurgent stronghold of Tadamon.
Residents said internet connections in the capital went down in the early afternoon and mobile and land telephone lines were only working intermittently, in what they said was the worst disruption to communication since conflict erupted last year.
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 1:30pm (local time) 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 the capital, 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, opposition activists said.
The past two weeks have seen military gains by rebels who have stormed and taken army bases across Syria, exposing Assad's loss of control in northern and eastern regions despite the devastating air power which he has used to bombard opposition strongholds.
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's Abdelrahman said that rebel units from around Idlib province massed early on Thursday morning to attack Wadi al-Deif, a large 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 have met fierce resistance from government forces, backed up by air strikes.
If Wadi al-Deif fell to rebels, who already control northern border crossings to Turkey, Assad would be dependent on a single land route - from the Mediterranean port of Latakia - to supply his forces fighting to win back Aleppo.
Assad is fighting an insurgency that grew out of peaceful protests 20 months ago and has escalated, after a crackdown, into a civil war in which 40,000 people have been killed.
Most foreign powers have condemned Assad but have said they will stop short of providing arms to rebel fighters as they fear heavy weapons could make their way into the hands of radical Islamist units, who have grown increasingly prominent.