A general view of Mosul Dam in northern Iraq September 20, 2014. REUTERS/Ari Jalal

BAGHDAD (Reuters) - The U.S. military has a contingency plan to deal with a potential collapse of Mosul dam in northern Iraq which would be catastrophic, the top U.S. general in Iraq said on Thursday.

U.S. Army Lieutenant General Sean MacFarland said Iraqi authorities understood “the potential” for the collapse of the hydroelectric dam, whose foundation requires constant grouting to maintain structural integrity.

He said the military was working with the government on a plan to protect Iraqi civilians from the impact of a collapse, which would send a surge of water down the heavily populated Tigris river valley.

“The likelihood of the dam collapsing is something we are trying to determine right now ... all we know is when it goes, it’s going to go fast and that’s bad,” MacFarland, head of the U.S.-led coalition bombing Islamic State in Iraq and Syria, told reporters in Baghdad.

“If this dam was in the United States, we would have drained the lake behind it. We would have taken that dam out of commission,” he added.

Islamic State seized Mosul dam in August 2014, raising fears they might blow it up and unleash a wall of water on Mosul and Baghdad that could kill hundreds of thousands. Kurdish Peshmerga fighters recaptured the dam two weeks later with the help of coalition airstrikes and Iraqi government forces.

While Islamic State militants are no longer a clear threat to the dam, coalition spokesman U.S. Army Col. Steve Warren said they had stolen equipment and chased away technicians.

“There was a steady grouting schedule that had been maintained for a long time. When that stopped, obviously the deterioration of the dam increased accordingly,” he said.

An Italian company, the Trevi Group, is finalising a contract with Baghdad to upgrade the 3.6-km (2.2-mile) long dam, which has suffered from structural flaws since it was built in the 1980s.