Syrian warplanes carried out 20 air strikes against the strategic rebel-held Yabrud area near Damascus on Wednesday, as the army escalated a campaign there, activists and a monitoring group said.

Troops meanwhile captured Jarajir, a village near Yabrud and on the Lebanese border, a military source told AFP. He denied the army had launched a new offensive, however, saying it was carrying out "routine" operations.

Amateur video distributed by activists showed plumes of black smoke rising above Yabrud's houses and yellow hills, as a fighter jet soars through a clear blue sky.

The army last year launched a wide-scale operation to retake a string of towns in the area, seizing towns along the highway between Damascus and central Homs.

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."

The Syrian Observatory for Human Rights, a Britain-based group relying on a network of sources inside Syria, confirmed the escalation, saying Yabrud was hit by 20 air strikes Wednesday.

Located in the Qalamoun mountains, Yabrud lies near the Lebanese border and on the Damascus highway to the country's third largest city Homs, which has suffered some of Syria's worst fighting in the past three years.

Despite being under rebel control, Yabrud was spared much of the violence engulfing most other opposition areas. Activists had for months considered it a haven for non-violent opposition to the 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.

According to the Observatory's Rami Abdel Rahman, troops and Hezbollah have been stepping up pressure on Yabrud for several weeks, frequently shelling the town, which is home to a mixed Muslim and Christian population.

A group of 12 Syrian and Lebanese Orthodox nuns taken by gunmen in early December from the nearby historic town of Maalula are believed to be in Yabrud.

Abdel Rahman said Hezbollah and the paramilitary National Defence Force were supporting the army in fighting against Al-Nusra Front and local rebels near Yabrud.

Six jihadists and rebels were killed in the fighting, with loyalists losing 11 troops, he said.

The violence forced families from Yabrud and nearby areas to flee into Lebanon's Arsal, just across the border, the UN refugee agency's Dana Sleiman said on Twitter.

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 in Aleppo has been widely denounced by rights groups as "indiscriminate."

In Daraa in the south, air strikes against rebel areas killed 22 people, including 19 civilians, 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.

The violence appears to have escalated since the launch of peace talks in Switzerland on January 22, with the Observatory reporting an average death toll of 236 people a day since then, the highest since Syria's uprising began.