The rights group said in a report that the Islamic State militant group had flagrantly violated that same law by deliberately putting civilians in harm's way to shield their fighters and impede the advance of Iraqi and coalition forces.
Iraqi Prime Minister Haider al-Abadi declared victory in Mosul on Monday, three years after Islamic State seized the city and made it the stronghold of a "caliphate" they said would take over the world.
A 100,000-strong alliance of Iraqi government units, Kurdish Peshmerga fighters and Shi'ite militias had launched the offensive in October, with key air and ground support from the international coalition.
Much of Mosul has been destroyed in grinding street-to-street fighting, thousands of civilians have been killed and nearly a million people fled their homes, according to the United Nations.
Amnesty said Iraqi forces and the coalition carried out a series of unlawful attacks in west Mosul since January, relying heavily on Improvised Rocket Assisted Munitions (IRAMs), explosive weapons with crude targeting capabilities that wreaked havoc in densely populated areas.
"Even in attacks that seem to have struck their intended military target, the use of unsuitable weapons or failure to take other necessary precautions resulted in needless loss of civilian lives and in some cases appears to have constituted disproportionate attacks," the report said.
Neither the Iraqi defense ministry nor coalition officials were immediately available to comment on the Amnesty report.
Amnesty also slammed Islamic State for a host of crimes that have been documented previously.
The militants rounded up residents in contested villages and neighborhoods and forced them to move into conflict zones in west Mosul for use as human shields, it said. As clashes neared, they trapped the civilians inside houses without access to food or medical care, it said.