Syria's air force launched air raids on Qara near the border with Lebanon on Sunday as loyalist forces tried to storm rebel positions in the town, a monitoring group said.
"Since the morning, the town of Qara has been hit by air strikes," said Syrian Observatory for Human Rights director Rami Abdel Rahman.
"Warplanes bombarded the town heavily yesterday (Saturday). Regime troops are trying to storm it and to drive the rebels out."
The Britain-based group said opposition fighters in the town were determined to resist despite the onslaught.
Pro-regime newspaper Al-Watan reported that "the army hit the Qalamoun mountains hard, closing in on the terrorists around Qara," using the government's term for rebels.
Violence in the Qalamoun area intensified on Friday.
Both the regime, backed by fighters from powerful Lebanese Shiite movement Hezbollah, and rebels including jihadists affiliated to Al-Qaeda have bolstered their forces in the area.
Qalamoun, which has a strong rebel presence, is strategic because it borders Lebanon and is used by rebels as a rear base for operations around the capital.
For President Bashar al-Assad's regime, it is important because it is on the road linking Damascus to the central province of Homs.
It also houses regime weapons depots.
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For months, Qalamoun mostly avoided the violence tearing other areas of Syria apart, but in past weeks parts of the town have been battered by near-daily shelling.
Lebanese authorities said 500 families fleeing Syria arrived in the border town of Arsal, in eastern Lebanon, on Sunday.
The latest influx brought the number of families who arrived in Arsal this weekend alone to 1,700, said Lebanon's social affairs ministry. Most had fled from Qara.
Arsal's municipality chief Ali al-Hojairi told AFP: "Many are sleeping in cars. They need shelter."
More than 800,000 Syrians fleeing their country's brutal civil war have taken refuge in Lebanon. Many suffer terrible shortages as local authorities and international agencies struggle to provide for them.
Elsewhere in Syria on Sunday afternoon, Damascus and several parts of the country's south were hit by a power outage for several hours, a minister and residents said.
"The power outage in the southern areas is the result of sabotage by armed terrorist groups against the high voltage cables that feed the southern areas," said Electricity Minister Imad Khamis.
Residents of the capital confirmed the news.
Power cuts have become a regular occurrence in Syria as the armed conflict that started in March 2011 as a rebellion against the Assad regime has intensified.
Also on Sunday mortar rounds slammed into neighbourhoods in central Damascus, killing at least two people, said the Observatory.
State media blame "terrorists" for the mortar attacks, which have increased in recent days.