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.
It is up to the "high command of (the Iraqi) armed forces to adopt the proper and right decision," al-Sistani said.
On the ground, the Iraqi troops pressed their push in Tikrit on Friday as fighter planes pounded IS targets from above. 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 authorized to talk to reporters.
Iraq's military suffered a humiliating defeat during the Islamic State group's lightning offensive last year, when it crumbled in the face of the group's onslaught in Iraq's second-largest city, Mosul.
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.
The militias also receive significant backing from Iran, one of Iraq's biggest allies, which raised the prospect of coalition interests uncomfortably overlapping with those of Iran.
At least two-thirds of the ground forces fighting in Tikrit were linked to Iraq's Popular Mobilization Forces, the official government-backed body made up of different militias. The remaining fighters include the military and Iraq's Sunni tribes, the latter of which the US has cited as a crucial component to fighting the Sunni militant group.
