The Syrian Observatory for Human Rights, a Britain-based monitoring group with a wide network of sources on the ground, could not confirm the use of napalm or white phosphorus.

But Observatory chief Rami Abdel Rahman said there had been a spike in burn wounds over the past two days, mostly casualties brought in from the Ras al-Ain area.

Kurdish officials posted a video on social media showing children with burns one doctor in Hasakeh province argues are consistent with the use of banned weapons.

Made notorious by their widespread use by US forces in the Vietnam War, napalm and related chemicals are mixtures of a gelling agent and a volatile petrochemical used in incendiary bombs.