Spokesman for the Russian Defence Ministry Igor Konashenkov gives a briefing on US strikes against Shayrat air base in Syria last April.

Russia accused the United States on Tuesday of pretending to fight Islamic State 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 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 defense ministry.