Russia's military says two US F-15 fighter jets have dropped phosphorus bombs over Syria's Deir al-Zor province, an allegation denied by the United States.
The Russian miliary said the strikes took place on Saturday, the TASS and RIA news agencies reported on Sunday.
The air strikes targeted the village of Hajin, the last major stronghold of Islamic State 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.
"None of the military units in the area are even equipped with white phosphorous munitions of any kind."
Human rights groups have said the US-led coalition against Islamic State 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.
The coalition or partner forces likely conducted aerial strikes in July on eastern Syria where remnants of Islamic State are holed up, the coalition has said.
US-Russian tensions have heightened in recent days as the Trump administration considers military options should Syria ignore US warnings against using chemical weapons in an expected assault on Idlib, the last big enclave of rebels opposing Syria's government.
Russian and Syrian jets resumed strikes in Idlib and Hama on Sunday as Damascus stepped up its assault after a Russian-Iranian-Turkish summit failed to agree on a ceasefire.
