But Nusra Front fighters and members of al-Qaida's Islamic State in Iraq and the Levant vowed to return to Qara soon, the Observatory added.
A reporter for the private Lebanon-based satellite channel Al-Mayadeen who is embedded with the Syrian army reported from inside the town that troops had begun dismantling mines and explosives planted by the rebels. The channel showed soldiers on patrol searching for booby traps.
The border offensive is part of a larger government push that started last month and has taken several objectives, including a string of opposition-held suburbs south of Damascus as well as two towns and a military base around the northern city of Aleppo.
Qara, some 100 kilometers (60 miles) north of Damascus, has borne the brunt of the offensive so far. On Monday, government warplanes and artillery pounded the town and surrounding countryside, according to the Observatory, which uses a network of activists to monitor the conflict.
Rebels have responded with shelling into Damascus, with mortar fire hitting almost every day over the past two weeks leaving dozens of people dead or wounded.
State-run news agency SANA reported that a mortar round hit the wall of the capital's historic Ummayad Mosque in the old quarter of Damascus, causing material damage but no casualties.
Some 120,00 people have been killed in the 2 year conflict, which started as peaceful protests but deteriorated into all-out civil war after a brutal government crackdown. Both sides have killed civilians.
Also on Tuesday, New York-based Human Rights Watch said opposition fighters had apparently executed civilians and others in their custody during an attack late October in the central Christian village of Sadad.
In a statement, it said other civilians "were also killed unlawfully by opposition sniper fire" as well "opposition shelling."
Opposition fighters also refused to allow residents of the village to leave their homes in areas with active fighting, HRW said, quoting residents. In at least one case, fighters allegedly used a resident as a human shield. The village was captured days later by government troops.
"Opposition fighters came into Sadad claiming they would not harm civilians, but they did just that," said Sarah Leah Whitson, Middle East director at Human Rights Watch. "There is no excuse for indiscriminate or targeted attacks against civilians or civilian sites."
Meanwhile in Russia, Foreign Minister Sergey Lavrov said the main Western-backed Syrian opposition, the Syrian National Coalition, has agreed to attend talks in Moscow along with other Syrian opposition groups.
Lavrov wouldn't say when the talks are set to be held. He added that the coalition chief, Ahmad al-Jarba, said he couldn't visit Moscow himself at the time when the talks are planned, but remains open for future contacts.
Russia has proposed to host the talks as part of diplomatic efforts to help convene a Syrian peace conference in Geneva. Lavrov said that the National Coalition's acceptance of the offer is a positive step that signals that the opposition is getting "more realistic."
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Mroue reported from Beirut.