The US and Iran have been cooperating in fighting ISIS forces in Iraq, sources in the country said.

Iraqi sources told the London-based Arabic-language newspaper Al-Sharq al-Awsat Thursday that the US and Iran have been cooperating in fighting ISIS forces in Iraq. The sources said that US and Iranian forces have “come to an understanding” on the deployment of forces to retake the city of Tikrit in the Salah-a-Din district from ISIS.

The sources said that Iran has sought to be in charge of the military operation to conquer Tikrit. Iraqi officials have been reluctant to ask for US air forces to bomb ISIS targets, possibly because they are seeking to defer to Iranian forces. There are numerous Iranian advisors in Iraq, led by Kassam Suleimani, the head of the Al-Quds Brigades of the Revolutionary Guards, the sources said.

Speaking before a Senate committee this week, US Army Commander Martin Dempsey said that Iran is more involved in Iraq than it has been since 2004. Iran has been supplying mortar shells and heavy rocket launchers to Iraqi armed forces, the sources said.

Dempsey added that two thirds of the forces fighting ISIS in Tikrit are made up of Iranian organized and backed militias, and the rest are Iraqi army troops. If, Dempsey said, the forces were able to wrest Tikrit from ISIS in a “clean” manner, “this would have a positive effect.”