"Two American F-15 planes carried out bombings on September 8 (Saturday) targeting the area of Hajin in the Deir Ezzor region using incendiary phosphorous ammunition," Russian General Vladimir Savtchenko said.

The air strikes targeted the village of Hajin, the last major stronghold of Daesh in Syria, and resulted in fires, but there was no information about casualties, the Russian military said.

A Pentagon spokesman denied that US planes dropped phosphorus bombs.

"At this time, we have not received any reports of any use of white phosphorous," said Commander Sean Robertson.

Human rights groups have said the US-led coalition has used white phosphorous munitions over the course of the Syria conflict.

The bombs can create thick white smoke screens and are used as incendiary devices. The rights groups criticise the use of the munitions in populated zones because they can kill and maim by burning people to the bone.

More recently, U.S.-led coalition forces have also been accused of improper use of white phosphorus in Iraq and Syria, according to Human Rights Watch.

Amnesty International has warned that white phosphorus can be dangerous even when not used as a weapon, because it can burn partially and reignite weeks later, posing a residual risk to civilians.