Human Rights Watch on Thursday condemned abuses by all sides in a days-long standoff in Iraq's Anbar province which has left more than 250 people dead and displaced 13,000 families.
"Apparently unlawful methods of fighting by all sides have caused civilian casualties and severe property damage," the New York-based rights group said in a statement.
It criticised Iraqi forces for using what it alleged was indiscriminate mortar fire in civilian neighbourhoods, and militants for deploying in and attacking from populated areas.
HRW also echoed concerns from the UN and other NGOs that a government blockade of Fallujah and Ramadi, the two largest cities in Anbar, was limiting access to key supplies of food, water and fuel.
Fallujah and parts of nearby Ramadi, both in the western province of Anbar, have been outside government hands for days -- the first time militants have exercised such open control in major cities since the insurgency that followed the 2003 US-led invasion.
The Iraqi Red Crescent said it had provided humanitarian assistance to more than 8,000 families across Anbar but that upwards of 13,000 had fled and were living with relatives, or in schools or other public buildings.
Some families have sought refuge in the neighbouring province of Karbala and, according to HRW, in Iraqi Kurdistan.
Fighting erupted near Ramadi on 30 December, when security forces cleared a year-old Sunni protest camp.
The violence spread to Fallujah, and militants moved in and seized the city and parts of Ramadi after security forces withdrew.
Both Ramadi and Fallujah were insurgent strongholds in the years after 2003, and Fallujah was the target of two major assaults in which US forces saw some of their heaviest fighting since the Vietnam War.
They eventually wrested back control of Anbar with the support of Sunni tribesmen who formed the Sahwa (Awakening) militias, which allied with US troops against Al-Qaeda from late 2006.
But Sunni militants have regained strength, bolstered by the war in neighbouring Syria and widespread Sunni Arab anger with the federal government.