Russia on Sunday accused the United States of carrying out bombing using white phosphorous in eastern Syria against military targets in civilian areas, which is banned under international law.
"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 in a statement.
"These strikes resulted in fierce fires. We are clarifying the information concerning possible deaths and injuries," he added.
In Syria, ISIS still controls a pocket in Deir Ezzor between the Euphrates River and the Iraqi border, and also remains in the vast Syrian desert.
An international coalition led by the United States is helping a coalition of Kurdish and Arab fighters from the Syrian Democratic Forces (FDS) which for months have been conducting an offensive to oust ISIS.
The Russian army, which has been intervening militarily since 2015 in Syria in support of the forces of its ally President Bashar al-Assad, was accused in March by the Syrian Observatory for Human Rights of having used incendiary bombs during the regime's offensive against the rebel stronghold of Eastern Ghouta near Damascus.
Moscow dismissed the allegation as a "shameless lie".
Use of white phosphorous is banned under the Geneva Convention for use against civilians and against legitimate military targets in areas with a major civilian population.