These gains have unnerved Tehran and has reportedly led to a number of purported airstrikes against ISIS by the Iranian Air Force. So far, the airstrikes carried out by the Iranian have focused in the ethnically and religiously mixed Iraqi province of Diyala, which butts up against Iran.
"Iran regards the area as a buffer zone and does not tolerate any military threats within that buffer zone," Hamid Reza Taraghi, a conservative Iranian politician, told the LA Times in Tehran. Iraqi officials had asked Iran "to be quiet about" it's involvement.
Most recently, an Iranian F-4 Phantom was caught on camera by Al Jazeera carrying out a bombing run in the end of November in the towns of Jalawla and Sadiya, although the footage has since been taken down. The two towns are each less than twenty miles from the Iranian border.
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The Iranian airstrikes have provided air support to Iraqi and Kurdish fighters, alongside Iranian-led Shiite militiamen, in chasing ISIS out from the two towns. Kurdish officials have stated that there was no US air support during the offensive against the Jalawla and Sadiya. The US says it refuses to directly coordinate military campaigns with Iran, although that's easier said than done.
"Although it is theoretically possible for Iranian planes to fly inside Iraq without any coordination with other air forces operating in the same airspace, it would be suicidal," Aviation expert David Cenciotti told Business Insider. "For proper deconfliction of tactical assets, prior coordination and air space management and control are required. There are several aircraft performing Airspace Control and Airborne Early Warning over Syria and Iraq: no plane could fly undetected in the area."
An unnamed US
"We are aware of that. I wouldn't say we're necessarily concerned with it - we kind of have our eyes on it," the official said. The official noted that the Iranian strikes occurred close to the Iran-Iraq border, away from where the US coalition has normally carried out airstrikes.
Despite Taraghi's insistence that Iran carried out attacks in Diyala against ISIS, other Iranian officials have denied any military involvement.
"Iran has never been involved in any air strikes against Daesh (Islamic State) targets in Iraq. Any cooperation in such strikes with America is also out of question for Iran," a senior official told Reuters on condition of anonymity.
Iraqi officials, meanwhile, note that they will take any help they can get.
"The U.S. is always our first choice" for military assistance, but "if we need to seek help elsewhere, we will," one Iraqi official told the LA Times.