The seizure of Deir Attiyeh, on the Damascus-Homs highway, comes two weeks into an army offensive in the Qalamoun region, important to the regime for its proximity to the capital and to the rebels for the supply lines it offers to neighbouring Lebanon.
It also comes amid intense international efforts to hold a peace conference aimed at ending the 32-month conflict that has killed more than 120,000 people and displaced millions.
But in the run-up to the January 22 talks in Geneva, Assad's forces appear to be pushing for leverage with as many battleground victories as possible.
"Our heroic army has taken total control of the town of Deir Attiyeh in Damascus province after it crushed the terrorists' last enclaves there," state television said.
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A high-ranking security official confirmed the report to AFP, saying the town had been "cleansed" and that "operations to expel the terrorists from nearby areas are ongoing."
The Syrian Observatory for Human Rights said the army was now in "near-total control" of Deir Attiyeh, which had been captured by the rebels last Friday, although gunfire could still be heard.
"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.
Also fighting in Qalamoun is Lebanese Shiite militant group Hezbollah, which backs Assad and has sent thousands of fighters into Syria.
Fighting raged elsewhere in Syria, a day after Iran said it and Turkey, which support opposing sides in the conflict, would press for a ceasefire ahead of the Geneva 2 peace conference.