The strongest warning about a possible collapse of the Mosul Dam came on Sunday from the Iraqi government and the US embassy in Baghdad.
In a statement released Sunday night, Iraqi Prime Minister Haider al Abadi asked residents along the Tigris river to move at least six kilometres (3.7 miles) away from its banks.
A US security message estimates that Mosul could be inundated by as much as 70 feet (21 meters) of water within hours of the breach.
"Prompt evacuation offers the most effective tool to save lives of the hundreds of thousands of Iraqis," stated the US embassy in Baghdad on Sunday, calling the dams risk of collapse serious and unprecedented.
He estimated that floodwaters could kill nearly 1.5 million Iraqis living along the Tigris.
In August 2014, DAESH terrorist group seized the dam, raising fears that they might blow it up and kill hundreds of thousands of people.
The dam was recaptured two weeks later by Iraqi government forces backed by air strikes from the US-led coalition, but the disruption of maintenance operations has increased the likelihood of a breach.
An Italian company has been awarded a contract to make urgent repairs to the dam, which has suffered from structural flaws since its construction in the 1980s and requires constant grouting to maintain structural integrity.
Iraq's minister of water resources said earlier this month there was only a "one in a thousand" chance the dam would collapse, and that the solution was to build a new dam or install a deep concrete support wall.