The United States has warned its citizens to be ready to leave Iraq in the event of what it has said could be a catastrophic collapse of the country's largest hydro-electric dam near Mosul.

Iraqi officials have sought to play down the risk but Washington urged its citizens to make contingency plans now.

A US security message  cited estimates that Mosul, which is northern Iraq's largest city and under control of Islamic State of Iraq and the Levant (ISIL) fighters, could be inundated by as much as 21 metres of water within hours of the breach.

More funds needed to save Mosul dam in Iraq

Cities downstream on the Tigris River such as Tikrit, Samarra and the Iraqi capital Baghdad could be inundated with smaller, but still significant levels within 24 to 72 hours.

"We have no specific information that indicates when a breach might occur, but out of an abundance of caution, we would like to underscore that prompt evacuation offers the most effective tool to save lives of the hundreds of thousands of people," the security message said.

Iraqi Prime Minister Haider al-Abadi said on Sunday that precautions were being taken, but described the likelihood of such a scenario as "extremely small".

ISIL seized the dam in August 2014, raising fears that they might blow it up and unleash a wall of water on Mosul and Baghdad that could kill hundreds of thousands.

The dam was recaptured two weeks later by Iraqi government forces backed by air strikes from a 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 was built in the 1980s and requires constant grouting to maintain structural integrity.

Hussein Hamad, the chief engineer of the Mosul dam maintenance department, told Al Jazeera last month the dam is not 100 percent secure .

"During the 1980s, foreign companies used to maintain the dam through drilling and [by] reinforcing the dam. That process was handed over to us afterwards," Hamad said.

"We are given cement and a number of excavators, but we need spare parts for the machines we are using for the ongoing maintenance process."

Iraq's minister of water resources said earlier this month that 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.

The US says Mosul could be inundated by 21m of water within hours of a dam breach [File: Khalid Mohammed/AP]

Source: Al Jazeera And Reuters