HRW said it did not know the size of Syria's incendiary weapon stockpile which it believed was supplied by the former Soviet Union.

Stephane Dujarric said Mr Ban was alarmed by reports of air strikes involving incendiary weapons and bunker-busting bombs.

The Geneva Protocol does not ban flame or incendiary weapons, but prohibits them from being used on or near concentrations of civilians.

Syria has not signed the 1980 Convention on Conventional Weapons which includes restrictions on the use of incendiary weapons in areas with large civilian populations.

Incendiary weapons contain flammable materials such as napalm, thermite or white phosphorous, which can set fire to buildings or cause severe burns and respiratory damage.