The spiritual leader of Iraq's Shiites has called on Iraqi forces battling to retake the city of Fallujah from Islamic State militants to protect civilians trapped there.

Grand Ayatollah Ali al-Sistani says that "saving innocent people from harm's way is the most important thing, even more so than targeting the enemy."

His comments were delivered at Friday prayers by his representative Ahmed al-Safi in the holy city of Karbala.

Earlier this week, Iraqi forces launched the offensive on Fallujah. The city is located 65 kilometers (40 miles) west of Baghdad.

Rights groups have expressed concerns for the tens of thousands of civilians estimated to still be in the city, which has been in IS hands for more than two years.