Syria's air force has used incendiary weapons in dozens of attacks over the last year, Human Rights Watch
said on Sunday, including a half-ton bomb which killed 37 people at a school in the northern province of Aleppo.
Calling on the world to condemn Syria's use of the weapons, which contain flammable substances designed to set objects on fire and cause burn injuries, it said international laws restricting their deployment also needed to be tightened.
"Syria has used incendiary weapons to inflict terrible harm on civilians, including many children," said Bonnie Docherty, an arms researcher at the organization, which will present its findings at an international meeting in Geneva this week.
Syrian President Bashar al-Assad is fighting a 2-1/2 year civil war with rebels seeking his overthrow. The crisis started with peaceful protests but escalated into armed conflict after his forces shot at demonstrators demanding change.
The scale of the violence on both sides has steadily escalated, with the authorities resorting to tanks and artillery, then helicopters, fighter jets and surface-to-surface missiles to strike at their opponents.
As well as incendiary weapons, Assad's forces have used cluster bombs and vacuum bombs and are accused by the West of firing rockets loaded with the nerve agent sarin into districts outside Damascus in August, killing hundreds.
"Other countries should condemn Syria's use of incendiary weapons just as they have its use of chemical weapons and cluster bombs," Docherty said.
HRW said that since last November, when it documented one of the first cases of incendiary bomb use in the Damascus suburb of Daraya, Syrian jets and helicopters had dropped incendiary bombs at least 56 times. All of the weapons were made in the Soviet Union, it said.
The organization quoted Saleyha Ahsan, a British emergency doctor who treated patients from the Aleppo school attack on Aug. 26, as saying most of them were covered in burns.
One victim arrived with what she described as 90 percent, third degree burns. "The clothes had been burned off him. It was the most horrific injury I have ever seen in a live patient. Only his eyes moved."
He died before he could be evacuated to Turkey, HRW said.
Incendiary weapons can contain any number of flammable substances, including napalm, thermite, or white phosphorus.
More than 100 countries - but not Syria - have signed up to an international convention banning their use in areas with "concentrations of civilians". But loopholes and inconsistencies limit its effectiveness, HRW said.
"The existing international law restricting the use of incendiary weapons could be strengthened in many ways," Docherty said. "But Syria's egregious incendiary attacks show that a global ban would be the best solution."