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 report.
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.
A rescue worker in Saraqeb said warplanes had dropped phosphorus bombs there, but he had not heard of the use of thermite. He said use of phosphorus was not a new development.
"It's normal, these are often used," said Laith Abdullah of the Syrian Civil Defence, also known as the White Helmets, a rescue group working in rebel-held areas.
Videos posted on social media purportedly from Saraqeb on Sunday showed flaming materials hitting the ground and spreading large fires.
The Observatory said thermite had first been used in the Syrian conflict in June 2016 by the Syrian government.
The missile strike was a response to what Washington said was a gas attack by Syrian warplanes in the town of Khan Sheikhoun in southern Idlib that killed scores.
Syria denies using chemical substances and denies it carried out the attack.
The Observatory reported Syrian warplanes took off from the same air base less than a day after the US attack and carried out air strikes on rebel-held areas.
