DAMASCUS, May 18 (Xinhua) -- At least 11 soldiers and pro-government fighters were killed Friday in five explosions that rattled the arms depot in the air base of the central province of Hama, a monitor group reported.

Tens of others were wounded and missing as a result of the explosions in the arms depot of Hama air base, said the Syrian Observatory for Human Rights. It added that the death toll could likely rise as some of the wounded are in critical conditions.

The UK-based watchdog group said it's not yet clear what caused the explosions in the arms depot and fuel storage area in the air base, noting that it could either be a foreign strike or a malfunction.

The observatory said the air base is used by the Syrian army for launching strikes on rebel-held areas in the countryside of Hama and the northwestern province of Idlib.

The state media outlets reported the explosions but stopped short of giving any more details.

Pro-government activists posted a video online showing the smoke rising from the air base.

Iranian media outlets said the blasts took place inside the arms depot near the base, adding that it was possible due to overheating and was not an attack.

The incident took place as the Syrian army has made new progress in Hama, retaking areas in the southern countryside after the evacuation of the rebels towards northern Syria.