Iraqi and Coalition forces fighting Islamic State (IS) militants in the Iraqi city of Mosul used unnecessarily powerful weapons, human rights group Amnesty International says.
They are accused of deploying heavy weapons in densely populated areas.
A Coalition spokesperson told the BBC that Amnesty's report on Mosul was "irresponsible and an insult".
The report also documented major abuses by IS, including the use of human shields and summary executions.
It said IS had deliberately killed hundreds - possibly thousands - of men, women and children attempting to flee, and left bodies to hang from electricity pylons as a warning to others.
Amnesty's report, The Civilian Catastrophe in West Mosul, says bombardments by Iraqi and US-led coalition forces killed a "huge number" of civilians.
It calls for an independent commission to investigate the number of civilian deaths.
Examining hostilities in the west of the city from January to mid-May, it accuses Coalition forces of having failed to adapt their tactics even when it was clear that IS militants had forced civilians into areas most likely to come under attack.
They used "imprecise weapons with wide-area effects in densely-populated urban environments" and subjected them to a terrifying barrage of fire, the report says.
The use of human shields by IS "does not lessen the legal obligation of pro-government forces to protect civilians", said Amnesty's Middle East Research Director, Lynn Maalouf.
US military spokesman Col Joe Scrocca rejected the allegations. It was, he said, "irresponsible and an insult to the 71 nations, as well as the thousands of troops who died trying to remove this organisation, so that the people of Iraq could be free again".
"We understand that the toll on civilians is horrible and they didn't choose to be here," he added, "but who stands up for those brave Iraqis who put their lives willingly on the line to save more than 900,000 civilians from ISIS' [IS] control?"
Amnesty allegations against Iraqi and Coalition forces
- They carried out a pattern of attacks which "appear not to have struck their intended military targets, instead killing and injuring civilians"
- Some civilian deaths and injuries appear to have resulted from an inappropriate choice of weapons or a failure to take necessary precautions to verify a target
- On 17 March a US air strike on Mosul's al-Jadida neighbourhood was carried out to kill two IS snipers but also killed more than 100 civilians
- The UK government's assertion that hundreds of RAF air strikes in and around Mosul have resulted in no civilian casualties "is at best implausible"
Amnesty allegations against Islamic State militants
- They carried out a systematic campaign of forced displacement in the Mosul area, moving thousands of civilians from neighbouring villages to be used as human shields
- They trapped people inside their homes by welding doors shut, rigging booby traps at exits and summarily killing hundreds - perhaps thousands - of people attempting to escape
- Militants hanged the bodies of civilians trying to flee from electricity pylons