The resulting "tsunami-like wave" would crush nearly a third of Iraq, flatten major cities and wipe out hundreds of thousands of those living under ISIS control.
Health officials have been unable to evacuate many of those in immediate threat as they reside under the control of the evil caliphate, also known as ISIS.
The US embassy in Baghdad said long-running disrepair meant "approximately 500,000 to 1.47 million people living along the Tigris" would probably not survive the impact of the projected flood wave unless they evacuated.
A breach in the country's biggest dam would unleash an "inland tidal wave" which could reach tens of feet in height and knock out Iraq's entire power grid.
Parts of the ISIS stronghold Mosul could be under 45 feet of water within hours of a dam breach.
The flood water would even reach Baghdad within three days and bring chaos to the Iraqi capital with "increased health hazards, limited to no mobility, and losses of homes, buildings, and services".
A US embassy official said: "Mosul Dam faces a serious and unprecedented risk of catastrophic failure with little warning.
"A catastrophic breach of Iraq’s Mosul Dam would result in severe loss of life, mass population displacement, and destruction of the majority of the infrastructure within the path of the projected floodwave.
"Much of the territory projected to be damaged by a dam breach is contested or ISIS-controlled, suggesting an authority-directed evacuation is unlikely, and that some evacuees may not have freedom of movement sufficient to escape.
"The lack of an authority-directed evacuation effort probably will result in some vulnerable groups - like the sick, disabled, and elderly - being left behind."
The 3.2-kilometer-long Mosul Dam holds back as much as 12.5 million cubic meters of water, according to a construction industry website.
A report dating back to 2006 even described the structure "the most dangerous dam in the world".
Repair works at the dam, built in 1984, have been neglected amid ongoing instability in the region.
ISIS militants seized the dam in August 2014 but US airstrikes helped Kurdish and Iraqi forces take back control of it the same month.
However, Mosul still remains in the hands of the militant terror group although ISIS faces an imminent onslaught of Iraqi and Kurdish forces.
Iraq's own prime minister has tried to play down the risk of a breach at the country's largest dam, calling the possibility "unlikely" and insisting "the situation is under control".
An advisor to the office of PM revealed a collapse of the Mosul Dam would be "a thousand times worse" than the impact of Hurricane Katrina.
The anonymous source even warned terrorists could weaponise the dam if the government flees the city.
He said: "If the (ISIS) offensive to take back Mosul goes well, there is a nightmare scenario that ISIS could itself strike the dam as they withdraw from Mosul," Express reported.
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