The Iranian-born Grand Ayatollah Ali al-Sistani also said he wanted the political blocs to agree on the next parliament speaker and president by the time the new legislature meets on Tuesday.
A cleric representing al-Sistani, Abdul-Mahdi al-Karbalaie, told worshippers in a Friday sermon at the holy city of Karbala that selecting the three before parliament meets would be a "prelude to the political solution that everyone seeks at the present."
Iraq's political leaders have been under growing pressure to form an inclusive government from the United States, Iraq's main Western backer, which hopes such a government would diminish support for the militants among the disaffected Sunni minority.
The reclusive al-Sistani is the most revered figure among Iraq's Shiites. A call up to arms he issued on June 13 prompted tens of thousands of Shiites to volunteer to join the security forces in the fight against the extremists.
Iraq's Shiite, Sunni and Kurdish leaders have been bickering among themselves for years, and it took them nine months to form a government after parliamentary elections in 2010.