Pro-Assad forces have clashed with rebels near the demilitarized zone in northwest Syria.

The fighting reportedly began with an attack by regime forces near the village of Halfaya in northern Hama Province early Friday. Unconfirmed claims said 22 members of the Jaish al-Izza faction were slain.

The jihadist bloc Hay’at Tahrir al-Sham claimed that it responded with an assault on the fortified camp used to attack the rebels. Posting pictures of slain pro-Assad troops, it said 25 were killed.

The Assad regime’s news agency SANA said the pro-Assad assault was “in response to [rebel] attacks on a military position with heavy machineguns”.

Other claims said regime forces shelled parts of the towns of Jarjanaz and Khan Sheikhoun and villages in Idlib Province on Friday.

Reliable pro-opposition sources said foreign militia, led by Iranian Revolutionary Guards personnel, were involved in the attacks.

If confirmed, the incident is one of the most serious challenges to the demilitarized zone announced by Russia and Turkey on September 17. The zone is 15 to 20-km wide and 100-km long around Idlib and northern Hama, where an estimated 3 million Syrians — about 20% of the country’s remaining population — live.

Pro-Assad forces have also shelled villages on the periphery of the zone, killing seven people in one attack and 10 in another last month.

Russia has rejected the Assad regime’s calls to suspend the zone, where Turkish personnel are alongside rebels. The agreement suspended a planned Russian-regime offensive to overrun the area, the last major opposition territory in Syria.