The Syrian Observatory for Human Rights war monitor said rebels in the zone fired several "mortar shells at an army position in nearby Hama province, killing two soldiers" late Saturday.

Also on Saturday, rebels shelled Aleppo province from other parts of the demilitarised area, with no casualties recorded, the Britain-based monitor said.

"This is the first clear violation of the deal since the heavy weapons were withdrawn. This area is supposed to be clear of heavy weapons, including mortar shells," said Observatory head Rami Abdel Rahman.

Under a deal reached by opposition backer Turkey and regime ally Russia last month, rebels and jihadists alike were meant to have pulled out all heavy arms from the horseshoe-shaped zone by October 10.

The deadline had reportedly been met, with Ankara, rebel factions, and the Observatory all reporting that the area was free of heavy-duty weaponry.

Syrian pro-government newspaper Al-Watan also reported on the apparent violation.

On Sunday morning it said that western parts of Aleppo province were being hit with "rocket fire and shelling with heavy weapons, which were supposed to be pulled out from the area".

And an AFP correspondent in western Aleppo reported mortar fire in the area after several days of quiet.