Iraq's most revered Shiite cleric is calling for unity among the country's forces battling the Islamic State group after major, Iran-backed Shiite militias pulled out of the offensive in the militant-held city of Tikrit in protest over U.S. airstrikes there.

Grand Ayatollah Ali al-Sistani's spokesman, Ahmed al-Safi, says the top cleric's message for Friday prayers was that such splits among Iraqi forces will have a negative impact.

According to al-Safi, al-Sistani says coordination between the military, Shiite militias and tribes is necessary for the success of the operation to retake Saddam Hussein's hometown and defeat IS.

On Thursday, most of the Shiite militias announced their boycott of the Tikrit operation, after the U.S. got involved and started providing airstrikes in support of the mission at the request of Iraq's government.