Syrian soldiers backed by Hezbollah fighters recaptured the town of Maaloula, north of Damascus, on Monday, military sources and state television said, further squeezing rebels' supply routes through the Qalamoun mountains into Lebanon.
Islamist fighters, some from the al-Qaida-linked Nusra Front, had taken over part of the ancient Christian town in December and held several nuns captive until releasing them in March in a prisoner-exchange deal.
Government forces have recaptured several rebel-held areas and border towns in recent months, closing off supply routes from Lebanon and securing the main highway leading north from Damascus toward central Syria, Homs and the Mediterranean.
"They are inside Maaloula now. It is under their complete control, including the monastery of Mar Thecla," said a military source in contact with government fighters on the ground.
At another ancient Christian site, Mar Sarkis - a Greek Orthodox monastery dating back to the fourth century and located on a hilltop above Maaloula - a Reuters witness saw crosses and pictures of saints smashed and covered with dust on the floor.
"God willing, the army will be able to free all of the Qalamoun area within days," a Syrian army officer in Maaloula told Reuters. "Most of the terrorists have been killed and others fled to nearby villages.
"We can say that 80 percent of Qalamoun is under the control of the Syrian army."
Syrian state television aired pictures of bodies it said were Nusra fighters killed during the offensive.
Maaloula, 5 km (3 miles) from the main road linking Damascus to Homs, has changed hands at least four times in attacks and counter-assaults by rebels and government forces.
Control of the road would help secure President Bashar Assad's grip over central Syria, and would also enable safe passage for hundreds of tonnes of chemical agents that are due to be shipped out of the country this year to be destroyed.
Assad, who appeared on the verge of defeat last year, said on Sunday that the conflict was at a "turning point" in his favor.
In Washington, State Department spokeswoman Jen Psaki rejected his view.
"Our analysis remains what it has been, that this is a war of attrition, and neither side has been able to deliver or hold onto significant gains," she told reporters.
In the last few weeks, Assad's forces have recaptured strategic towns along the Lebanese border, including Yabroud and Rankous near Maaloula.
Maaloula's churches and monasteries attracted both Christian and Muslim pilgrims before the conflict. Some of its inhabitants still speak Aramaic, the language of Christ, and the monastery of Mar Thecla has a reputation among believers for miraculous cures.
Syria's Christian community, about 10 percent of the population, is wary of the rising power of Islamist groups within the rebel movement.
Only a small percentage of Christians have taken up arms in the civil war that broadly pits minorities, in particular Assad's Alawite sect, an offshoot of Shi'ite Islam, against the Sunni Muslim majority.