Syrian and Russian warplanes have repeatedly used incendiary weapons in "disgraceful" attacks on civilians in northern Syria, Human Rights Watch charged Tuesday.
The rights group said it had documented the use of incendiary weapons at least 18 times since June that had resulted in more than a dozen injuries.
There was "compelling evidence" that Russia was supporting Syrian government planes in those attacks, the New York-based watchdog said.
"The Syrian government and Russia should immediately stop attacking civilian areas with incendiary weapons," said HRW arms director Steve Goose.
"The disgraceful incendiary weapon attacks in Syria show an abject failure to adhere to international law restricting incendiary weapons," he said.
When dropped from aircraft, incendiary weapons leave distinctive trails of explosives in the sky and trigger small, intense fires upon contact.
They were used widely during the Vietnam war and are banned by the UN Convention on Certain Conventional Weapons.
HRW documented attacks with such weapons since early June, including two cases on August 7 that hit opposition-controlled parts of the cities of Aleppo and Idlib.
"I could clearly see the flames bursting," said Idlib resident Mohammad Taj Al-Din Othman, who supplied HRW with photos of the attack.
"Within 10 minutes, there were more strikes. The fire was unbelievable, it turned night into day."
A civil defense volunteer told HRW: "The fire took over everything, houses, cars, oil tanks, and even grass."
The Syrian Observatory for Human Rights monitoring group said it too had documented the use of thermite -- a type of incendiary substance -- by Russian planes in Idlib, Aleppo and Deir Ezzor provinces, and Raqa.
And earlier this month, activists in Daraya, a besieged rebel-held town near Damascus, accused the regime of using banned chemical agent napalm against residents there.
All sides of Syria's complex war have exchanged accusations of attacks against civilians and use of unconventional weapons including chlorine and mustard gas.
HRW said the use of incendiary weapons in Syria had "increased significantly" since Russia began its air war in support of Damascus on September 30, 2015.
In a letter to HRW in November, Russia acknowledged that "improper use" of incendiary weapons had resulted in "significant humanitarian damage" in Syria.
Since 2012, HRW has documented the use of four different incendiary weapons in Syria, all manufactured by the former Soviet Union.
More than 290,000 people have been killed and millions forced to flee their homes since Syria's conflict erupted in March 2011.