Syrian government forces bombarded one of two remaining rebel holdouts in the strategic Qalamoun region on Friday, a day after taking a key town in the area north of Damascus.
Troops are trying to encircle rebels near Damascus, pushing to capture the Qalamoun region and sever rebel supply lines across the border with Lebanon.
Government soldiers bombed parts of the town of Nabak, one of two rebel holdouts in the region.
The town of 55,000, including a Christian minority, lies 80 kilometres (50 miles) north of Damascus, and an NGO said troops backed by Lebanon's Shiite group Hezbollah had moved in on Thursday night.
The Syrian Observatory for Human Rights said the army was shelling rebel positions that had halted its advance.
The army has "apparently decided to use overwhelming force," Observatory director Rami Abdel Rahman said.
The advance into Nabuk comes a day after the army secured the nearby town of Deir Attiya, and follows its recapture of the strategic town of Qara, which led to thousands of refugees flooding into neighbouring Lebanon.
A Syrian security source said the army had only Nabuk, nearby Yabroud and a handful of surrounding villages left to capture before securing the Qalamoun region completely.
"If this town is captured, all we'll have left is Yabroud and some other villages to completely block off the border with Lebanon and to stop any entrance or exit of rebels into Lebanon," the source said.
"The next phase will be to retake the south (of Syria). The north and the east are for later," he added, referring to areas under the control of the rebels or of Kurdish militia.
The string of towns, which the army has gradually captured in a southward sweep towards Damascus, lie along the strategically important Damascus-Homs highway.
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The highway has been blocked by the fighting, preventing the delivery of fuel to Damascus from the Homs refinery and causing shortages in the capital.
Qalamoun has been a key rear-base for the rebels, who have used the mountainous region to store arms and as a transit point for fighters.
The army's operations in Qalamoun, as well as an attack against the rebel stronghold of Eastern Ghouta outside the capital, have been bolstered by outside support.
Both Lebanon's Shiite Hezbollah group and the Iraqi Shiite Abul Fadl al-Abbas Brigade are fighting alongside the regime.
According to the Observatory, Hezbollah has lost some 17 fighters and the Abbas Brigade another 11 in the operations in the past two weeks.
A pro-Hezbollah site published the photos of 11 "martyrs."
A source close to Hezbollah said Thursday that a nephew of Lebanese Agriculture Minister Hussein Hajj Hassan, a prominent Hezbollah figure, was killed on Wednesday along with three comrades.
The fighting comes as the international community pushes the Syrian regime and opposition to come to the table for peace talks in Geneva on January 22.
The Syrian regime has said it will attend the talks, but insists that President Bashar al-Assad's departure from office will not be on the table.
The key Syrian opposition National Coalition has welcomed the setting of the date as "very positive" but insists Assad can have no role in Syria's future.
It also wants to see humanitarian aid delivered to Syria, the release of prisoners and "an immediate end" to massacres before the talks begin.