Rebels detonated 60 tonnes of explosives packed underneath a large Syrian army base, blowing a hillside hundreds of meters into the air, an insurgent in the operation who provided footage of the attack said on Thursday.

The casualty toll from the blast was not immediately known.

An Islamic Front commander said his brigade dug a 850-meter (2,800-foot) tunnel underneath Wadi al-Deif base, which is surrounded by rebels but has remained inside government control for the entirety of the three-year-long civil war.

Footage provided by the commander of the base, which stretches over a large area of land, showed the ground balloon up before breaking into a cloud of earth that engulfs the area.

The commander, who spoke on condition of anonymity, said he did not know how many government soldiers had been killed but said it would help rebels break into the base, which has been used for attacks in the surrounding province of Idlib.

"Another attack like this and we won't even need to move in to take the base," he said via Skype, adding that once rebels take the base they would control all of the south Idlib, which is situated in Syria's northwest bordering on Turkey.

Rebels fighting to overthrow President Bashar al-Assad regularly carry out guerrilla attacks but have only started using large tunnel bombs in recent weeks on military targets, including a hotel used by soldiers in Aleppo last week.

The government boasts far superior firepower and its forces killed more than 40 people, many of them civilians, in air strikes on Wednesday, a monitoring group said.

Fifteen people were killed, including three from an emergency medical team, during five air raids in Atarib in the northern province of Aleppo, the Britain-based Syrian Observatory for Human Rights said early on Thursday.

Four rebel fighters were killed by air strikes in the same area, while 21 people, including women, were killed in air strikes on the Sarmada area in Idlib, the anti-Assad monitoring group said.

Gunbattles, air strikes, car bombs, shelling and executions regularly kill more than 200 people a day in Syria, where a conflict that started as a peaceful protest movement has killed over 150,000 people and forced millions from their homes.

Despite the carnage and loss of swathes of territory in the north and east to insurgents, Syria plans to hold a presidential election next month that is all but certain to give Assad a third term. Opponents have dismissed the vote as a farce.