Iraq's top Shia Muslim cleric urged government forces battling to retake Falluja from Islamic State militants to spare civilians trapped in the city on Baghdad's western approaches.

Grand Ayatollah Ali al-Sistani's appeal reflected concerns that a large civilian death toll in the battle for the mainly Sunni Muslim city could aggravate sectarian tensions in Iraq. The Baghdad government has been led by Shias since the 2003 fall of Saddam Hussein, a member of the Sunni minority.

Sistani added his voice to many calls for restraint in the battle launched on Monday to retake Falluja, the first Iraqi city to fall under the control of the ultra-hardline Sunni Islamic State (IS), in January 2014.

"Sayyid Sistani reaffirms his recommendations that the ethics of jihad (Islamic holy war or struggle) be respected," his representative, Sheikh Abdul Mahdi al-Karbalai, said in a statement.

"Don't be extreme ... don't be treacherous. Don't kill an old man, nor a boy, nor a woman. Don't cut a tree unless you have to," he said, citing sayings of the Prophet Mohammed.

Aid agencies have become alarmed about civilian suffering in a city that has been under siege for six months, and the United Nations has urged combatants to protect inhabitants trying to escape the fighting.

APPEAL FOR CIVILIANS' SAFE PASSAGE

On Wednesday morning Iraqi troops concentrated artillery fire on Falluja's northern and northeastern neighborhoods, according to a resident contacted via the Internet.

A Falluja hospital source said that six civilians were killed and 11 wounded on Wednesday morning, raising the overall death toll since Monday's launch of the government offensive to 35 - 21 civilians and 14 militants.

"Fierce fighting is now raging around the city," Save the Children said in a statement on Wednesday, calling for safe civilian exit routes to be established as quickly as possible.

Falluja's population is around 100,000, according to U.S. and Iraqi government estimates.

The offensive is part of a government campaign to roll back Islamic State's seizure of wide tracts of northern and western Iraq. Baghdad's forces retook Ramadi, the Anbar provincial capital near Falluja, in December but have not yet tackled a bigger challenge - IS-held Mosul, Iraq's largest northern city.

An Iraqi military spokesman said troops were trying to tighten the encirclement of Falluja by advancing on the western front, near the village of Khalidiya.

The Association of Muslim Scholars of Iraq, a hardline political organization formed after Saddam's ouster to represent Sunnis, has condemned the assault on Falluja as "an unjust aggression, a reflection of the vengeful spirit that the forces of evil harbor against this city".

Sistani wields enormous influence over Iraq's Shias. It was at his call that Shia militias regrouped in 2014 in a coalition known as Hashid Shaabi (Popular Mobilization), to stem Islamic State's stunning advance through the north and west.

Hashid Shaabi will take part in encircling Falluja but will not enter the city unless the Iraqi army fails in doing so, said Hadi al-Amiri, the leader of the Badr Organisation, the largest component of the Shia coalition.

"Our decision is to encircle the city from the outside and let the security forces operate; if the security forces are unable to cleanse the city, we will then go in,'' he said, according to video recording on the state-run TV channel.

By Reuters