Grand Ayatollah Ali al-Sistani's appeal came a day after the militias, which had been instrumental so far in the operation to recapture Saddam Hussein's hometown, announced their boycott of the Tikrit offensive.
The US got involved in the operation and started providing airstrikes on Wednesday in support of the mission at the request of Iraq's government.
Al-Sistani said that coordination between the military, Shiite militias and tribes is necessary for the success of the operation, according to his representative Ahmed al-Safi in the holy site of Karbala.
On the ground, the Iraqi troops pressed their push in Tikrit today as fighter planes pounded IS targets from above.
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Militants holed up in the center of Tikrit fired mortars at the military, slowing its progress despite the new aerial campaign.
A senior military commander told The Associated Press that roadside bombs and booby traps planted by the Islamic State militants demanded extreme caution.
The commander spoke on condition of anonymity because he was not authorised to talk to reporters.
Days after the fall of Mosul, al-Sistani called on volunteers to rush to the battlefields and reinforce the military, and many of the country's militias reported for duty. But with a range of different leaders and loyalties, many of them became difficult to control.
The militias' pullout has prompted mixed reactions in Iraq. While several of them have been accused by human rights groups of committing atrocities against Sunni civilians, many in Iraq view them as the most capable fighting force in the country today.