Military spokesman Igor Konashenkov said the Syrian regime was attempting to push the jihadists out of eastern Deir Ezzor province, but that arrivals from Iraq were boosting their numbers.
"The US-led coalition, pretending to fight IS, largely in Iraq, sees all this but continues allegedly active measures against IS in Syria for some reason," he said.
"The continuing arrival of terrorists from Iraq raises serious questions about the anti-terrorist objectives of the US air force and the so-called 'international coalition'."
"Is this change in approach from the US and the coalition a bid to cause maximum disruption to the Syrian army, backed by the Russian air force, as it seeks to free Syrian territory to the east of the river Euphrates?"
Over the past month Moscow has repeatedly accused the US of hindering the Russian-backed Syrian army offensive in the east of the country.
Russia has been flying a bombing campaign in Syria since 2015, when it stepped in to support President Bashar al- Assad's rule and tipped the conflict in his favour.
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