Iraq's most influential Shiite cleric has urged the government to step up support for the Shiite militias battling the Islamic State group alongside the Iraqi military.

Grand Ayatollah Ali al-Sistani's comments, relayed by a representative during Friday prayers in the Shiite holy city of Karbala, come as Iraqi forces backed by the militias are carrying out a large-scale offensive to retake IS-held Sunni city of Tikrit.

Al-Sistani's spokesman Ahmed al-Safi also urged authorities to pay outstanding salaries to many Shiite militiamen, adding that such delays were "unacceptable."

The Iranian-backed Shiite militias have played a crucial role in regaining territory from the Islamic State group, supporting Iraq's embattled military and police forces in battle. U.S. officials estimate that as many as 20,000 militiamen may be involved in the ongoing Tikrit operations.