The Syrian army launched an intense bombardment Friday of one of the last areas under rebel control in the Qalamoun mountains north of Damascus, a monitoring group said.
Shelling of rebel positions in the town of Nabuk began before dawn, the Syrian Observatory for Human Rights said.
The army "apparently decided to use overwhelming force," Observatory director Rami Abdel Rahman said.
"Fierce battles are under way in Nabuk, which regime forces backed by Lebanese Hezbollah fighters entered on Thursday."
The Syrian army is trying to capture the Qalamoun region, on the border with Lebanon, in a bid to sever rebel supply lines.
On November 19, troops seized the town of Qara, and on Thursday they announced the capture of nearby Deir Attiya, before moving on to Nabuk.
A Syrian security source said the recapture of Nabuk would leave only the Yabrud area and a few surrounding villages outside of government control.
"If this town is captured, all we'll have left is Yabrud and some other villages to completely block off the border with Lebanon," the source said.
Abdel Rahman said Nabuk had been besieged by the army since Qara's fall on November 19.
"Neither food nor medicine has been able to enter Nabuk since then," he said.
Qara, Deir Attiya and Nabuk all lie along the main highway between Damascus and Syria's third largest city Homs, adding to their strategic value for the government.
More than 120,000 people have been killed in Syria since the conflict erupted in March 2011, and millions more have fled their homes.