The road to the airport in the Syrian capital Damascus has been closed and flights cancelled amid reports of fierce clashes with rebels in the area.
The government appears to be mounting an unprecedented offensive against rebel-held districts in the east of the city, BBC correspondents report.
The clashes came as internet and phone systems in the country went down.
The Syrian government has previously cut off access to the internet during major operations.
However, correspondents say a nationwide switch-off is unprecedented.
But Syria's information minister said "terrorists" had cut off the internet and that engineers were working to repair the fault.
Emirates airline and Egypt Air have cancelled flights to Damascus.
Government sources said a plane carrying 150 staff from the UN peacekeeping force had taken off from Damascus airport on Thursday evening.
A replacement team had already arrived earlier in the day, the sources said.
State TV reported on Thursday evening that the airport road had been "secured" after military intervention. But correspondents say the highway remains closed and there are more reports of fighting.
It is believed that Syrian government reinforcements were sent into the area after rebels fired mortars at the runway, the BBC's Paul Wood in Beirut reports.
A source in the Free Syrian Army told our correspondent the attack on the airport had been planned for a long time and that rebel fighters had been "inching closer and closer" over the past few days and hours.
The aim of the plan was to capture the airport and not just conduct a hit-and-run attack, the source said.
The rebels carrying out the attack were all from Eastern Ghouta, a suburb of the capital, and were well armed after capturing government weapons, including heavy weapons, in recent weeks, the source added.
Our correspondent says there have also been reports of passengers still being trapped inside the terminal and at an airport hotel. No rebels appear to be inside the airport.
UN peacekeepers injured
Despite the communications blackout, the BBC was able to get through to residents in the centre of Damascus, who said they could hear and see what they believed to be the biggest army offensive so far against rebel-held districts.
The offensive appeared to be going on in the east of the city, extending to the airport 27km (17 miles) south-east of the centre.
The main road to the airport passes through rebel-held territory which has regularly been the target of government air strikes.
The Syrian Observatory for Human Rights (SOHR), a UK-based activist group, also reported a major government offensive, saying clashes were most intense in the suburb of Babbila, near the rebel stronghold of Tadamun.
The group, whose information cannot be independently verified, said there were clashes all along the road.
State TV said government forces were fighting "al-Qaeda elements", mostly in the suburbs of Duma and Daraya.
Two Austrian soldiers from the United Nations Disengagement Observer Force (UNDOF) in the Golan Heights were wounded as their convoy came under fire on the road to the airport. Their injuries were not life-threatening, the Austrian defence ministry said.
The Syrian ambassador in Vienna was to be summoned to the foreign ministry to explain what happened, Austrian government officials said in an official statement.
"Syria is responsible for the safety of our [UN] soldiers and must ensure that they can fulfil their international mandate", they said.
Meanwhile the UN and Arab League envoy to Syria, Lakhdar Brahimi, said he had "the elements of a peace plan" but could not move ahead without the backing of the Security Council.
"No doubt what is very, very urgently needed is a ceasefire that can hold and that, I believe, will require a peacekeeping mission," he said on Thursday.
"It bears repeating that the situation is bad in Syria and getting worse, that unfortunately the parties themselves are not ready to have an internal solution.
"The region is also not really capable, at this time, of helping for a peaceful solution. The place where a peaceful solution can be initiated is the Security Council."
On Wednesday, two car bombs hit Druze and Christian areas, also to the south-east of Damascus, killing 34 people.
No group has said it was behind the bombings, and there was no immediately obvious military or government target, reports the BBC's Jim Muir in Beirut.
Activists say more than 40,000 people have been killed since the uprising against President Bashar al-Assad began in March 2011.