The spiritual leader of Iraq's Shiite majority is calling on doctors from across Iraq to help civilians fleeing clashes in the latest fight against the Islamic State group in Iraq.

Grand Ayatollah Ali al-Sistani says medical workers should travel to areas around the battle for Tal Afar to help "to treat the wounded and treat them as a humanitarian, national and religious duty."

The call during the Friday sermon delivered by Sistani's representative Sheikh Abdulmehdi al-Karbalai, from the holy city of Karbala comes after the operation to retake Tal Afar began last week. Tal Afar sits west of Mosul, where victory was declared against IS in July. Thousands of civilians are estimated to have been killed in the nine-month fight.

Iraqi defense officials say about 10,000 civilians remain inside the city.