Iraq's largest dam is in danger of collapse and could putting hundreds of thousands at risk in the cities of Mosul and Baghdad.

US experts have warned that the Mosul dam in northern Iraq could buckle under the water pressure, according to The Washington Post, which based its report on assessments by the US Army Corps of Engineers and other US officials.

"The Mosul dam is judged to have an unacceptable annual failure probability," said the Army Corps of Engineers draft report, according to the Post.

It is "the most dangerous dam in the world," read the report, scheduled to be released Tuesday.

As many as 500,000 civilians in Mosul and Baghdad were at risk from any deluge, dam manager Abdulkhalik Thanoon Ayoub told the Post.

A 27 million dollar US reconstruction project to help strengthen the dam has been marred by incompetence and mismanagement, according to a report by a US oversight agency, the Post reported.

Top Iraqi officials rejected a US proposal to build a second dam downstream, saying it is unecessary.

The debate between US and Iraqi officials has taken place behind closed doors because they fear frightening Iraq citizens, according to the Post.

"Mosul Dam is 'unsafe' in any definition," read segments of one presentation obtained by the Post.

It continued: "Condition continually degrading" and "Failure mode is credible," and warned of possible "Mass civilian fatalities."