Activists claimed the attack was in retaliation for the Russian and Syrian government defeat in southern Aleppo over the weekend.

The use of air-dropped incendiary bombs on civilian populations would be a violation of the Convention on Certain Conventional Weapons, of which Russia is a signatory.

The Syrian government, which was also reported to have carried out airstrikes on the northwestern city on Sunday, is not a party to the Convention.

Because of the flammable content, incendiary weapons cause excruciatingly painful burns and start fires that are hard to extinguish.

Thermite is the hottest burning man-made substance in the world.

A model of incendiary bomb believed to contain thermite was seen loaded onto an Su-34 fighter-ground attack aircraft at Russia's Hmeymim air base in Syria, in footage aired by RT, Russia's state-owned English language television channel, in June.

Russia's state-owned Sputnik news agency reported that Russian and Syrian aircraft on Sunday night dropped “heavy munitions” on Idlib, which they called a base for the “terrorist” group Jabhat Fateh al-Sham.