MOSCOW: Russia accused the United States on Tuesday of pretending to fight Daesh (ISIS) and of deliberately reducing its air strikes in Iraq to allow the group's militants to stream into Syria to slow the Russian-backed advance of the Syrian army.

In the latest sign of rising tensions between Moscow and Washington, the Russian Defense Ministry said in a statement that the U.S.-led coalition had sharply reduced its air strikes in Iraq in September when Syrian forces, backed by Russian air power, had started to retake Deir al-Zor Province.

"Everyone sees that the U.S.-led coalition is pretending to fight [Daesh], above all in Iraq, but continuing to allegedly fight Daesh in Syria actively for some reason," said Major-General Igor Konashenkov, a spokesman for Russia's defense ministry.

The result, he said, had been that militants had moved in large numbers from Iraqi border areas to Deir al-Zor where they were trying to dig in on the left bank of the River Euphrates.

"The actions of the Pentagon and the coalition demand an explanation. Is their change of tack a desire to complicate as much as they can the Syrian army's operation, backed by the Russian air force, to take back Syrian territory to the east of the Euphrates?," asked Konashenkov.

"Or is it an artful move to drive Daesh terrorists out of Iraq by forcing them into Syria and into the path of the Russian air force's pinpoint bombing?"

He said Syrian troops were in the midst of trying to push Daesh out of the city of al-Mayadin, southeast of Deir al-Zor, but that Daesh tried daily to reinforce its ranks there with "foreign mercenaries" pouring in from Iraq.