The announcement on Saturday from the Iraqi military cited "the increasing concern from Russia about thousands of Russian terrorists committing criminal acts within ISIS."
The news comes amid U.S. concerns about Russia's recent military buildup in Syria
and would appear to confirm American suspicions of some kind of cooperation between Baghdad and Moscow.
A U.S.-led coalition has been carrying out an aerial bombing campaign against ISIS positions in Iraq and Syria since last year. But now Russia is upping its presence in the region.
U.S. President Barack Obama is scheduled to meet with Russian President Vladimir Putin on Monday for the first time in nearly a year to discuss Moscow's plans in Syria.
Russia has sent aircraft, tanks and missiles into the war-wracked country, according to the Pentagon.
The United States had also been watching intelligence indicators that the Russians might be setting up some type of operation in Baghdad to coordinate their efforts with Iraqi, Iranian and Syrian elements.
"We remain committed to working closely with Iraq to defeat ISIL," Col. Steve Warren, spokesman for the U.S. coalition, said in a statement, using another acronym for ISIS. " As a sovereign nation, Iraq has relations with many nations and the agreements the Iraqis take that work to our common objective are up to them."
He added: "We recognize that Iraq has an interest in sharing information on ISIL with other governments in the region who are also fighting ISIL. We do not support the presence of Syrian government officials who are part of a regime that has brutalized its own citizens."
This wouldn't be the first time that the fight against ISIS has resulted in cooperation between U.S. allies and rivals.
Iraq's government has strong ties with Iran, which has been training, advising and supporting Iraqi Shiite militias
that are fighting ISIS.
And Australia announced in April
that it would share some intelligence with Iran, notably on fighters from Australia traveling to Iraq to join ISIS.