Major-General Igor Konashenkov, Spokesman for Russian Defense Ministry. Thomson Reuters MOSCOW (Reuters) - 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.

The Pentagon strongly denied the accusations, saying that the U.S.-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 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.

The result, he said, had been that militants had moved in large numbers from Iraqi border areas to Deir al-Zor where they were trying to dig in on the left bank of the River Euphrates.

"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?"

Operation Inherent Resolve spokesman US Army Col. Ryan Dillon, right, and Iraq armed forces spokesman Gen. Yahyah Rasul. (AP Photo/Karim Kadim) In Washington, Pentagon spokesman Colonel Robert Manning described the Russian accusations as "absolutely false".

"We remain committed to killing ISIS and denying them safe havens and the ability to carry out strikes in the region or globally," Manning said, using an acronym for Islamic State.

Konashenkov added that Syrian troops were in the midst of trying to push Islamic State out of the city of al-Mayadin, southeast of Deir al-Zor, but that IS tried daily to reinforce its ranks there with "foreign mercenaries" pouring in from Iraq.

(Reporting by Andrew Osborn. Additional reporting by Idrees Ali in Washington; Editing by Dmitry Solovyov and Gareth Jones)