Mainly-Shia militia in Iraq have retreated from a threat to boycott the offensive against the Islamic State in the city of Tikrit, as the country’s leading cleric called for cooperation in the fight.

The militia have been angered by US aerial intervention, including surveillance flights and bombing of the Islamic State, which began on Wednesday. On Thursday, four of the largest groups — with about 1/3 of the 30,000 men in the offensive — said they were considering the boycott, and one threatened to attack American forces.

However, Grand Ayatollah Ali Sistani used his Friday Prayer to call for unity in the offensive. His spokesman summarized:

The security commanders in the field must get together and talk with the supreme command of the armed forces to always take the right decision. Having disagreements among the different sides could have bad results on military operations.

Last summer, Sistani issued a fatwa for popular mobilization committees to help Iraqi forces push back the lighting advance of the Islamic State — including its capture of Tikrit, about 160 km (100 miles) north of Baghdad — across northern and eastern Iraq.

Rafid Jabouri, a spokesman for Prime Minister Haidar al-Abadi, said discontent had been quelled:

The essential part of the mandate for the popular mobilization committees was the fatwa that was issued by Ayatollah Sistani….

Everything was explained to the popular mobilization committees, so the decision to seek the assistance of the coalition was explained to them, and the Prime Minister’s instructions were clear.

The Prime Minister is the commander-in-chief of the armed forces, and the popular mobilization committees should be operating under the commander-in-chief, in full cooperation with the Iraqi armed forces.

The cooperation appeared to hold, despite the accident of an American airstrike that reportedly killed at least three federal policemen and killed and wounded an undetermined number of militiamen late Thursday.

The Badr Brigades, the largest Shia militia, said it was still in the offensive despite its opposition to the US intervention. “We haven’t retreated from our positions near Tikrit,” said Mueen al-Kadhumi, a Badr leader. “We won’t lift the siege against ISIS [Islamic State] so that they can escape; we will chase them and kill them after the Americans stop the airstrikes in Tikrit.”

The three-week offensive made initial advances near and in Tikrit but has been stalled for more than two weeks as the Islamic State continues its resistance in the city.