Amnesty International said on Tuesday that Iraqi forces and the US-led coalition fighting the so-called "Islamic State" (IS) in Mosul repeatedly violated international humanitarian law and may have committed war crimes.

The rights group also said that IS flagrantly violated international law by using thousands of civilians as human shields and not allowing them to evacuate. It also killed hundreds or even thousands of civilians who attempted to flee.

Read more: Iraqis wonder what will follow IS in Mosul

Amnesty recognized IS' tactics created "particular challenges" for pro-government forces.

"Iraqi government and US-led coalition forces failed to adequately adapt their tactics to these challenges – as required by international humanitarian law – with disastrous consequences for civilians," Amnesty said in a report named "At any cost."

Iraqi Prime Minister Haider al-Abadi declared victory in Mosul on Monday, nearly 10 months after a mix of Kurdish fighters, Iraqi government forces and Shiite militia launched an offensive to root out IS from Iraq's second largest city.

Much of Mosul, particularly in the dense Old City, has been destroyed as Iraqi government forces engaged in grueling street-to-street fighting backed up by coalition air support. According to the UN, about 920,000 people were displaced in fighting and few have returned to their homes.

Amnesty said pro-Iraqi government forces used unsuitable weapons for urban combat, including Improvised Rocket Assisted Munitions.

"With their crude targeting abilities, these weapons wreaked havoc in densely populated west Mosul, where large groups of civilians were trapped in homes or makeshift shelters," Amnesty said.

Read more: Will victory over IS bring peace to Mosul?

It is unclear how many civilians died directly or indirectly as a result of the battle for Mosul. Airwars, a monitoring group, documented 3,706 civilian deaths caused by attacks from Iraqi government forces or the coalition between February and June.

Amnesty said this figure likely drastically underestimates the true casualty toll as it is difficult to monitor deaths and assign responsibility.

Rebuilding Mosul and helping civilians remains a huge challenge. Aid groups warn that Iraq's humanitarian crisis will continue and funding is running low.

The military victory against IS also raises the prospect that a number of political rivalries and disputes that were put aside during the fight are likely to come to the fore again. 

IS took control of Mosul in June 2014 and declared a "caliphate" as it expanded through Iraq and Syria, prompting a US-led intervention that later expanded from Iraq to Syria.  

The group has been pushed back in large parts of Iraq and Syria, but even after its defeat in Mosul it is likely to continue to pose a long-term security challenge and change as both a movement and insurgency.

In Syria, the US-backed Syrian Democratic Forces are trying to retake Raqqa, IS' self-declared capital.

There are widespread reports that coalition airstrikes and street-to-street fighting are causing civilian casualties and destroying parts of the city as IS uses similar tactics as it did in Mosul.