Beirut - At least 30 Syrian soldiers were killed in a powerful explosion caused by a massive bomb that rebels planted in a tunnel they dug under a checkpoint, a monitor said on Tuesday.

“Around 30 regime forces were killed... in a blast in which several tons of explosives were planted in a tunnel beneath a checkpoint on the northeastern outskirts of the town of Maaret al-Numan,” group said the Syrian Observatory for Human Rights.

The Observatory said the tunnel used in Monday's bomb attack in the northwestern province of Idlib extended more than 200 metres.

A video posted online by Syria's biggest rebel alliance, the Islamic Front, showed an enormous explosion that threw dust and debris dozens of metres up into the air.

Opposition fighters have regularly made use of tunnels to facilitate their movement between areas and to attack government facilities, detonating explosive charges under checkpoints and headquarters.

Observatory director Rami Abdel Rahman said the checkpoint was one of the lines of defence for the Wadi Deif military base, one of the last regime strongholds in Idlib.

Opposition fighters have been trying to capture Wadi Deif and the nearby Hamadiyeh military base for more than a year, but the regime has thus far repulsed the attacks. - Sapa-AFP