Syria's air force carried out a dozen air strikes against the strategic rebel-held Yabrud area near Damascus for the first time on Wednesday, activists and a monitoring group said.
"There were 13 air strikes against Yabrud and its surroundings today," said the Syrian Observatory for Human Rights, without providing immediate details on casualties.
On the ground, an activist who gave his name as Amer told AFP via the Internet that "the (army's) campaign to take Yabrud has started. The air strikes are accompanied by an attempted ground offensive."
Located in the Qalamoun mountains, Yabrud lies near the border with Lebanon and on a strategic road linking Damascus to the war-torn country's third city Homs, which has suffered some of Syria's worst violence in the past three years.
While Yabrud is under rebel control, it had been mostly immune to the violence engulfing most other opposition areas. Activists had for many months considered it a haven for non-violent opposition to Syria's regime.
But recently, the jihadist Al-Nusra Front moved into Yabrud as the loyalist army and its Lebanese Shiite ally Hezbollah stepped up fighting in other areas of the Qalamoun mountains, taking over smaller villages from rebel control.
According to the Observatory's Rami Abdel Rahman, troops and Hezbollah have been upping the pressure on Yabrud for several weeks, frequently shelling the town, which is home to a mixed Muslim and Christian population.
On Wednesday, Hezbollah and the paramilitary National Defence Force were backing the army in fighting against Al-Nusra Front and local rebels in the Rima area near Yabrud.
Amateur video distributed by activists showed plumes of black smoke rise above Yabrud's houses and golden hills, as a fighter jet soars through a clear blue sky.
The violence forced families from Yabrud and nearby Flita and Jreijer to flee into Lebanon's Arsal, just across the border, the UN refugee agency UNHCR's Dana Sleiman said via Twitter.
A group of 12 Syrian and Lebanese Orthodox nuns taken by gunmen in early December from the historic town of Maalula were believed to have been taken to Yabrud.
Elsewhere, the death toll from air raids on Aleppo's rebel-held Sakhur neighbourhood on Tuesday rose to 27, including nine children, said the Observatory.
The air force's use of explosive-packed barrel bombs has been widely denounced by rights groups as "indiscriminate."
In Daraa in the south, air strikes against Tafas village killed nine people, including six children, said the Britain-based Observatory.
More than 136,000 people have been killed in Syria's nearly three-year war, and millions more forced to flee their homes.