BAGHDAD (AP) — Iraq's top Shiite cleric urged the government on Friday to press ahead with reforms, but cautioned that the anti-corruption campaign must not come at the expense of the fight against the Islamic State group and should remain within the boundaries of the law.

In a sermon delivered by a deputy, Grand Ayatollah Ali al-Sistani said the government had no choice but to continue the reforms and must swiftly go after the "big thieves of public funds."

The Iranian-born cleric also cautioned against the reform campaign distracting authorities from the fight against the Islamic State, describing it as the "most honorable and righteous battle because it is fought in defense of our existence and future."

"The battle for reform must not distract us from that fight," he said in comments delivered at the holy city of Karbala, south of Baghdad, by cleric and al-Sistani representative Ahmed al-Safi.

His warning against distracting authorities from the anti-IS fight appeared inspired by the struggle of the military and its allies to regain territory captured by the extremist group in the vast Anbar province west of Baghdad and its recent advances on the southern parts of the oil refinery town of Beiji, liberated from IS control last November.

Also, Iraq's second largest city, Mosul, has been under IS control since it was captured in June last year in a blitz that gave the group control of about a third of Iraq and neighboring Syria.

Shiite Prime Minister Haider al-Abadi has won parliamentary support to trim his government and vowed to fight corruption, amid rising popular discontent over poor services. He also declared his intention to seek a popular mandate to amend the constitution, adopted in 2005. "It is incomplete," he said.

Al-Sistani's comments came several hours before tens of thousands of Iraqis were expected to gather in a central Baghdad square and elsewhere in Iraq to resume a campaign of street protests to push for reforms and an improvement in the government's erratic services, from power and water to medical care and education. The protesters, who have for weeks gathered peacefully every Friday, are also calling for effective, anti-graft measures.

"The battle for reforms we are fighting these days is also a vital one that will determine our and the nation's future. We, people and government, have no choice but to win that battle," al-Sistani said.