Moscow accused the US on Tuesday of reducing air strikes against the Islamic State group in Iraq to let jihadists into Syria and fight the Russian-backed Syrian army, claims the Pentagon denied.

Russian 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.'"

The US-led coalition sharply reduced its strikes on Iraq in September, as Syrian forces were beginning to retake Deir Ezzor, Konashenkov said in a statement.

"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?" he asked.

Pentagon spokesman Colonel Rob Manning flatly denied the Russian claims and pointed to online tallies that show no let up in the bombardment of IS targets in Iraq or Syria.

"That's absolutely false," Manning said.

"We remain committed to killing ISIS and denying them safe havens and the ability to carry out strikes in the region or globally."

On Monday, the US-led coalition conducted seven strikes against IS targets in Iraq, a military website showed.

Manning also urged "all forces" to focus their efforts on beating IS.

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.

Source: AFP