Fighters of the Islamic State of Iraq and the Syria (ISIS) celebrate on vehicles taken from Iraqi security forces, at a street in the city of Mosul, June 12, 2014. Reuters

Iraq's highest-ranking Shia cleric, Grand Ayatollah Ali Sistani, has issued a religious summons to the nation's Shia Muslims to help defend the nation against the Sunni-led Islamic State of Iraq and Syria (ISIS), the New York Times reported Friday.

Sistani's fatwa aims to mobilize Iraq's majority Shia population against the jihadi group, which has gained control of Mosul and is threatening to raid Karbala and Najaf.

Shia Muslims are particularly worried that ISIS, which regards them as infidels, will trash some of Shia Islam's holiest sites in southern Iraq.

“Iraq and the Iraqi people are facing great danger, the terrorists are not aiming to control just several provinces, they said clearly they are targeting all other provinces including Baghdad, Karbala and Najaf," the statement from Sistani said. “So the responsibility to face them and fight them is the responsibility of all, not one sect or one party. The responsibility now is saving Iraq, saving our country, saving the holy places of Iraq from these sects."