BEIRUT, Nov 17 (Reuters) - Syrian forces are besieging a key town in the Qalamoun mountains, activists said on Sunday, a region used by rebels to cross from Lebanon and which links the capital to government strongholds along the coast.

Activist video footage showed artillery damage in the small highway town of Qara, roughly 50 km (30 miles) north of Damascus. The United Nations says that around one thousand families fled Qara for the Lebanese frontier on Saturday as the army closed in.

But U.N. refugee agency spokeswoman Dana Sleiman said there were no more families crossing the border on Sunday. This could be a result of closed army checkpoints around Qara.

A battle for the Qalamoun area, predicted for months by observers on both sides, could lead to unrest in neighbouring Lebanon as refugee numbers rise and Sunni Muslim anger grows against the Lebanese Shi'ite Hezbollah group which is helping crush the majority Sunni revolt.

Forces loyal to President Bashar al-Assad have used sieges to root out rebels from residential areas during the civil war that has cost more than 100,000 lives and displaced millions.

Sieges cost less in terms of money and casualties and have been a preferred method for the Syria army, even as civilians are trapped.

Damascus says the residents of besieged areas around Syria are "held hostage" by terrorists. Reuters was unable to get an official statement on Sunday regarding the siege of Qara.

Over one million Syrians are trapped in areas where aid deliveries have stalled, the United Nations says.

On Sunday, the army opened a checkpoint to civilians in the southern Damascus suburbs of Beit Sahem, a rare opportunity to escape a months-long siege for civilians.

Footage posted on the Internet showed hundreds of civilians on foot and some in cars piled high with luggage escaping the city. (https://www.youtube.com/watch?v=Rov9T95FG9I&feature=c4-overview&list=UUqsVMQlAh52iZZ7NDT1-amQ)

It was not clear why they were allowed to leave but local humanitarian groups have at times negotiated ceasefires to allow civilians to leave.

The Qalamoun advance is the latest of several military offensives that have put opposition forces on the defensive.

State news agency SANA said that the armed forces took control of more areas on the southeast approach to rebel-held Aleppo, in the north.

Despite losses, activist video footage showed that insurgents were continuing to attack Base 80 near Aleppo airport, which they lost to the army last week. (Reporting by Oliver Holmes)