Moscow has accused Washington of using white phosphorus in bombing raids in Syria. This comes as Russia's ally, Syria, reportedly drops barrel bombs on civilians in Idlib.
Russia's military said on Sunday that the US had carried out bombing attacks on military targets in civilian areas in Syria's Deir el-Zour province using white phosphorus, according to the TASS and RIA news agencies.
"Two American F-15 planes carried out bombings on September 8 targeting the area of Hajin in the Deir el-Zour region using incendiary phosphorus ammunition," Russian General Vladimir Savtchenko said in a statement.
"These strikes resulted in fierce fires. We are clarifying the information concerning possible deaths and injuries," he said.
The jihadi "Islamic State" (IS) group, which the US under President Donald Trump has pledged to wipe out, still holds sway in a small area of Deir el-Zour.
The Geneva Conventions ban the use of bombs with white phosphorus against civilians and against legitimate military targets in areas where the population is mostly civilian.
Russia, which is giving military assistance to its ally Syrian President Bashar Assad in combating an insurgency, was itself accused in March by the Britain-based Syrian Observatory for Human Rights of using incendiary bombs in an offensive against the rebel stronghold of eastern Ghouta near the Syrian capital, Damascus.
Moscow has described the allegations by the group, which is monitoring the conflict in Syria via a network of activists on the ground in the country, as a "shameless lie."
Moscow's accusations against Washington come as Syrian regime and Russian aircraft on Sunday flew air strikes for a second day on the country's last rebel bastion of Idlib.
Residents and rescuers said that Syrian army helicopters had dropped barrel bombs on villages in southern Idlib, killing at least two children, while Russian jets were reported to have carried out raids on rebel positions in the neighboring province of Hama.
The Syrian army has denied using barrel bombs, which typically contain high explosives and shrapnel, but UN investigators have found extensive evidence to bear out the accusation.
UN officials said on Monday that more than 30,000 people had been displaced by the bombardment of Idlib since it began last week.
Turkey and Western countries have warned of a humanitarian catastrophe if a major Russian-backed bombing offensive is launched on the region of Idlib and adjacent areas, which are home to almost 3 million people, many of them displaced by fighting elsewhere in the country. The UN has warned that such a campaign could drive up to 800,000 people from their homes.
Syria's civil war, which has its roots in peaceful protests against Assad's rule in 2011, has killed more than 350,000 people and displaced millions more.