Syrian troops backed by fighters from Lebanon's Hezbollah on Sunday launched an assault to oust rebels from the foothills of the Qalamun mountains north of the capital, state television said.
Elsewhere in the capital, fighters were due to withdraw "within hours" from the Palestinian refugee camp of Yarmuk following a truce struck on Saturday evening to end deadly fighting, a Palestinian official said.
Regime forces took parts of the strategic Qalamun region near the border with Lebanon in April, but some 2,000 rebel fighters withdrew to the hills from where they have launched guerrilla attacks.
Fourteen fighters from the powerful Shiite Hezbollah group have been killed in the area over the past two weeks, the Syrian Observatory for Human Rights said.
Syria's state broadcaster showed footage of the troops, saying they had taken "some hills overlooking the Rankus plain and are pursuing terrorist groups who tried to infiltrate the region from Lebanon".
An officer interviewed by the channel said "the army has cut the route off to terrorist groups who try to return to the region from time to time. The operation is continuing until the whole of the Qalamun area has been cleansed.
"This land will be a cemetery for all terrorists who decide to return."
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President Bashar al-Assad's regime refers to the armed opposition inside the country as "terrorists", without distinguishing between different groups.
Observatory head Rami Abdel Rahman said that "rebels hiding in the heights and in caves have been attacking army and Hezbollah positions for weeks after being pushed out of the region, which also prompted regime forces to withdraw".
He said the rebels had launched a counter attack "and succeeded in retaking their positions and expanding their presence".
Since the Syria conflict erupted in March 2011, more than 162,000 people have been killed and millions displaced.
Meanwhile fighters were expected to exit the embattled Yarmuk camp in southern Damascus "within hours", a Palestinian official told AFP in the Syrian capital on Sunday.
Anwar Abdel Hadi, political director of the Palestine Liberation Organisation in Syria, said "the armed men will pull out of the camp, checkpoints will be dismantled and rubble removed".
The pullout is part of a truce agreed between the PLO and gunmen "with the approval of the Syrian government", he said, adding that the fragile ceasefire went into force at 1500 GMT Saturday.
In mid-February, a truce paved the way for the withdrawal of fighters from Al-Qaeda's franchise in Syria, Al-Nusra Front, from Yarmuk, but fighting flared again in March.
The jihadists had accused regime forces besieging Yarmuk of violating the earlier ceasefire agreement.
Once home to 150,000 Palestinians as well as Syrians, Yarmuk has been under total army siege for the past year.