Iraq's top Shi'ite Muslim cleric is urging the government and allied Shi'ite militia forces fighting to retake Fallujah from Islamic State militants to spare trapped civilians.
Grand Ayatollah Ali al-Sistani's appeal on Wednesday reflected concerns that a large civilian death toll in the battle for the mainly Sunni Muslim city could kindle increased sectarian strife in Iraq.
The Baghdad government has been led by Shi'ites 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 begun on Monday to retake Fallujah, the first Iraqi city to fall under the control of the ultra-hardline IS, in January 2014.
Aid agencies are 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.
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Save the Children said severe shortages had driven the price of a single can of infant milk up to $US50 ($A69.62) at times, making it unaffordable for the parents of thousands of children.
On Wednesday, Iraqi troops concentrated artillery fire on Fallujah's northern and southern neighbourhoods, according to a resident contacted via the internet.
A Fallujah hospital source said the overall death toll since Monday's launch of the government offensive had risen to 43, made up of 26 civilians and 17 militants.
Up to 100,000 people have remained in the city, according to US and Iraqi government estimates.
Save the Children said up to 50,000 were being prevented from leaving Fallujah.
Sistani wields enormous influence over Iraq's Shi'ites.
It was at his call that Shi'ite militias regrouped in 2014 in a coalition known as Hashid Shaabi, to stem Islamic State's stunning advance through the north and west.
Hashid Shaabi will take part in encircling Fallujah 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 Shi'ite coalition.
