Syrian state media on Saturday cited a hospital in government-held Hama as saying 21 people suffered choking symptoms from poison gas after rebels shelled a village.
A war monitor, the Britain-based Syrian Observatory for Human Rights, also said 21 people were hospitalised with choking symptoms, but it was not known if this was from chemicals or smoke and dust raised by the shelling.
State news agency SANA cited the head of the Saqilbia National Hospital as saying the attack took place in the village of al-Rasif and it published images and footage of people lying in hospital beds wearing oxygen masks.
The area is near the frontline between the Syrian government and the last major rebel enclave in the northwest where army bombardment has escalated in recent weeks despite a Russian-Turkish deal to stop fighting.
On Friday and Saturday, airstrikes on rebel-held areas in the northwest killed 15 people including four children and injured 25 others the Observatory said.
The Organisation for the Prohibition of Chemical Weapons (OPCW), a global watchdog, has documented systematic use of nerve agent sarin and chlorine duringSyria's eight-year conflict.
From 2015-17, a joint U.N.-OPCW team was appointed to find blame in gas attacks and said Syrian government troops had used both chemicals several times. It also found that Islamic State had used sulfur mustard gas.
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The OPCW is looking into an alleged gas attack in November in government-held Aleppo that made up to 100 people ill, and which Damascus and its ally Russia blamed on insurgents.