Syrian or Russian warplanes dropped incendiary bombs on areas of Idlib and Hama provinces just days after a deadly gas attack in the region, activists and a monitoring group reported on Monday.
Moscow and the Syrian army were not immediately available for comment.
The British-based Syrian Observatory for Human Rights said Russian jets had used an incendiary substance called thermite in bombs they dropped over the towns of Saraqeb in Idlib and al-Latamenah in Hama, further south, on Saturday and Sunday.
More so, air strikes had reportedly killed at least 18 people including five children in a town in Syrian rebel-controlled Idlib on Saturday.
The death toll from the air strikes in Urum al-Joz was expected to rise, said the Observatory, a Britain-based organization that reports on the war using a network of sources in Syria.
The Civil Defence, which operates in rebel-held areas, said on a Twitter feed that the two air raids on the town on Saturday afternoon had killed 19 and wounded 22.
Scores of people were killed in a suspected chemical attack on Tuesday in the Idlib town of Khan Sheikhoun.
The United States launched a cruise missile attack on a Syrian air base in response to the attack. The Syrian regime has denied any responsibility.