The US embassy in Baghdad has warned the risk of the Mosul Dam collapsing is “serious and unprecedented” and has urged people to be ready to evacuate.
Maintenance work was disrupted after the dam was briefly seized by militants from so-called Islamic State in 2014.
If the dam burst, floodwaters could kill 1.47 million Iraqis living along the River Tigris, the embassy said.
Iraq’s prime minister has said precautions are being taken, but that such a scenario is “highly unlikely”.
The dam, Iraq’s largest has suffered from structural flaws since its completion in 1984, with the water constantly eating away at the soluble gypsum base on which it is built.
To counter the erosion, engineers need to drill holes in the gypsum and fill them with a cement grout mixture six days a week.
The United States will continue to support Prime Minister Abadi and the Iraqi government in their efforts to address this critical challenge.
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