Russia has accused the United States of pretending to fight Islamic State and deliberately hindering its efforts to destroy the militant group.
Moscow said Washington was 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.
The Pentagon strongly denied the accusations, saying that the US-led coalition fighting Islamic State posts data every day on the number and result of strikes for the public to see.
In the latest sign of rising tensions between Moscow and Washington, the Russian Defence Ministry said in a statement that the US-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 Islamic State, above all in Iraq, but continuing to allegedly fight Islamic State in Syria actively for some reason," said Major-General Igor Konashenkov, a spokesman for Russia's defence ministry.