Sistani, himself born in Iran, called "on all Muslims and free people around the world to denounce" the war and "stand in solidarity with the Iranian people."

He urged "all countries, particularly Islamic nations, to make every possible effort to end it immediately and to reach a just and peaceful solution to the Iranian nuclear issue."

Despite his non-Iraqi roots, Sistani has been a vital figure in Iraq's recent history, guiding his followers through decades of dictatorship, occupation and conflict.

But his influence extends beyond Iraq with millions of followers across the Muslim world.

Sistani always had a complex relationship with his birthplace, Iran, and was known for resisting Tehran's growing influence in Iraq.

That partly stems from the centuries-old rivalry between the holy Iraqi city of Najaf and Iran's Qom, the other main seat of Shiite religious authority.

Sistani has been seen as a competitor to Qom, and while Iran is ruled by clerics, Sistani has always avoided any formal role in government.

On Sunday, Sistani mourned Iran's supreme leader Ali Khamenei following his death in US and Israeli strikes, and urged Iranians to remain united to "thwart the aggressors' sinister goals".