The United States is pretending to fight the Islamic State [IS] group, Moscow alleged, claiming Washington has deliberately reduced airstrikes in Iraq in an attempt to allow the militants to flow into Syria, where the Russian-backed Syrian army continues to advance against IS.

The US-led coalition sharply reduced its airstrikes in Iraq in September when Syrian forces, backed by Russian air power, launched a campaign to recapture Deir az-Zour Province, Russia’s Defence Ministry said in a statement.

“Everyone sees that the US-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.

Military spokesman Igor Konashenkov said the Syrian regime was attempting to push the extremists out of the eastern Deir az-Zour province, but arrivals from Iraq, which he blamed on the US-led coalition, were boosting their numbers.

“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 Islamic State terrorists out of Iraq by forcing them into Syria and into the path of the Russian air force’s pinpoint bombing?”

Moscow has repeatedly accused the US of hindering the Russian-backed Syrian army offensive in the east of the country over the past month.

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, prompting thousands of deaths in the war-torn country.

The latest comments came days as Damascus confirmed it trapped a large group of IS militants in Deir az-Zour, after regime troops entered the eastern Syrian city of al-Mayadeen, a former stronghold of the militants.

The city, which was once a military hub for IS, is now one of the group’s last strongholds in eastern Syria’s Deir az-Zour province.

IS have still managed to launch several counter-attacks, killing scores of Syrian fighters and capturing two Russian men – believed to be mercenaries – in an attempt to cut off supply routes for pro-Damascus forces.

Meanwhile, US-backed fighters say they are nearing the “final week” of their assault to drive the Islamic State group from its one-time Syrian bastion Raqqa, as the militants’ self-described caliphate crumbles.