50 killed in air strikes on Syria's Al-Bab: NGO

On the political front, Syria's deputy foreign minister insisted that no solution to Syria's conflict proposed at a January 22 peace conference in Geneva would be implemented without President Bashar al-Assad's approval.

The Syrian Observatory for Human Rights said that helicopters on Sunday dropped explosive-laden barrels on the rebel-held town of Al-Bab in northern Syria's Aleppo province, killing at least 24 people, including two women and four children.

Their deaths in a market area of Al-Bab came a day after similar aerial bombardments on the same town that killed at least 26 others.

State news agency SANA, reporting on the conflict in the town, said "Syrian army units destroyed the headquarters of an Islamic (rebel) tribunal in Al-Bab in a special operation," without giving further details.

Large swathes of Aleppo province have been under rebel control for more than a year, and have suffered frequent aerial attacks by the regime.

Meanwhile in Damascus, children escaped a mortar attack on the French school, officials said.

"A mortar shell landed on the chimney of a classroom around 9:00 am (0700 GMT). No one was hurt but the windows shattered and the walls cracked," school receptionist Bashir Oneiz told AFP.

Aline Farah, a nurse at the Charles de Gaulle school in the upscale central district of Mazzeh, said students were in class when the mortar struck.

'Miracle that no one was hurt'

"It was a miracle that no one was hurt, neither students nor teachers nor employees," she said.

"They were all crying. They were terrified. We took them to an underground shelter," said Farah.

The attack was condemned by French Foreign Minister Laurent Fabius, who issued a statement calling it "cowardly".

The school, which is the only foreign school still open in the Syrian capital, has about 220 students, down from a pre-war figure of 900.

It caters both to Syrian students and the children of a few remaining foreigners living in the country despite a brutal 32-month conflict between the government and rebels.

Rebels in rear bases on the outskirts of Damascus regularly fire rockets and mortar rounds into the centre of the capital, with the attacks becoming more frequent and more deadly in the past few weeks.

Elsewhere, the Observatory said at least five regime forces were killed overnight in a suicide car bomb attack at a police checkpoint on the Damascus-Homs highway near the contested town of Nabuk.

The group said a fighter from the jihadist Al-Nusra Front carried out the attack, which came as regime forces try to gain control of Nabuk, as part of an operation to recapture the Qalamoun region.

The army has already captured the towns of Qara and Deir Attiyeh, and a Syrian security source told AFP troops now controlled 60 percent of Nabuk, which lies further south towards the capital on the Damascus-Homs highway.

The Observatory said regime warplanes carried out air strikes on the town on Sunday, and fierce fighting continued there between rebel fighters, including jihadists, and army troops backed by a pro-regime militia and Lebanon's Shiite Hezbollah.

The Syrian regime is trying to sever rebel supply routes that run through the mountainous Qalamoun region, north of Damascus, across the nearby border with Lebanon.

Amid the escalation of violence, Syria's Deputy Foreign Minister Faisal Muqdad said no solution proposed at peace talks next month would be implemented without Assad's blessing.

"The Syrian (government) delegation at Geneva will be working under Assad's directives, and any solutions proposed will have no impact unless Assad approves of them," Muqdad said in remarks reported in the Syrian press.

Under a US-Russian-led initiative, rebels and the Syrian government are scheduled to attend January 22 peace talks dubbed Geneva 2.

The opposition has agreed to attend the talks on condition that they lead to a transitional phase that excludes Assad and his regime.

But government officials and their backers in Iran and Russia insist there should be no preconditions, and Assad has also said he would be willing to stand for re-election in 2014.