Iraqi engineers involved in building the Mosul dam 30 years ago have warned that the risk of its imminent collapse and the consequent death toll could be even worse than reported.

They pointed out that pressure on the dam’s compromised structure was building up rapidly as winter snows melted and more water flowed into the reservoir, bringing it up to its maximum capacity, while the sluice gates normally used to relieve that pressure were jammed shut.

The Iraqi engineers also said the failure to replace machinery or assemble a full workforce more than a year after Islamic State temporarily held the dam means that the chasms in the porous rock under the dam were getting bigger and more dangerous every day.

On Wednesday, the Iraqi government announced it had signed a €273m (£210m) contract with an Italian contractor to reinforce and maintain the Mosul dam for 18 months, following talks in New York between the Italian foreign minister, Paolo Gentiloni, and US and Iraqi officials. Italy has said it plans to send 450 troops to protect the dam site, but it is unclear how long it will take to replace damaged machinery and reassemble the required workforce.

Iraqi PM and US issue warnings over threat of Mosul dam collapse Read more

The engineers warned that potential loss of life from a sudden catastrophic collapse of the Mosul dam could be even greater than the 500,000 officially estimated, as they said many people could die in the resulting mass panic, with a 20-metre-high flood wave hitting the city of Mosul and then rolling on down the Tigris valley through Tikrit and Samarra to Baghdad.

One of the Iraqi engineers, now living in Europe, described as “ridiculous” the Iraqi government’s emergency policy of telling local people to move 6km (3.5 miles) from the river banks.

Nasrat Adamo, the dam’s former chief engineer who spent most of his professional career shoring it up in the face of fundamental flaws in its construction, said that the structure would only survive with round-the-clock work with teams filling in holes in the porous bedrock under the structure, a process known as grouting. But that level of maintenance, dating back to just after the dam’s construction in 1984, evaporated after the Isis occupation.

“We used to have 300 people working 24 hours in three shifts but very few of these workers have come back. There are perhaps 30 people there now,” Adamo said in a telephone interview from Sweden, where he works as a consultant.

“The machines for grouting have been looted. There is no cement supply. They can do nothing. It is going from bad to worse, and it is urgent. All we can do is hold our hearts.”

At the same time as the bedrock is getting weaker and more porous, the water pressure on the dam is building as spring meltwater flows into the reservoir behind it. Giant gates that would normally be used to ease the pressure by allowing water to run through are stuck.

“One of them is jammed, and when one of them is closed the other one has to be closed. They must work together,” Adamo said. “Otherwise, you get asymmetric flow and that speeds up the erosion.”

Nadhir al-Ansari, another Iraqi engineer from when the dam was built, also voiced concern about the rising waters in the reservoir.

“The fact that the bottom outlets are jammed is the thing that really worries us,” said Ansari, now an engineering professor at the Luleå University of Technology in Sweden. “In April and May, there will be a lot more snow melting and it will bring plenty of water into the reservoir. The water level is now 308 metres but it will go up to over 330 metres. And the dam is not as before. The caverns underneath have increased. I don’t think the dam will withstand that pressure.

“If the dam fails, the water will arrive in Mosul in four hours. It will arrive in Baghdad in 45 hours. Some people say there could be half a million people killed, some say a million. I imagine it will be more in the absence of a good evacuation plan.”

He said the government policy response, calling on the local population to move at least 6km from the river Tigris, was “ridiculous”. The US embassy in Baghdad has urged American citizens to leave the area.

“What are all these people, millions of people, supposed to do when they get 6km away? There is no support for them there. Nothing to help them live.”

The Mosul dam was first conceived in the 1950s, but its construction was postponed because of the problematic geology of that section of the Tigris, where much of the bedrock is water-soluble. It was finally built by Saddam Hussein’s regime and seen at the time as a prestige project. At the time, Ansari was a scientific consultant at the irrigation ministry.

Nobody knows when it will fail. It could be a year from now. It could be tomorrow. Nasrat Adamo, the dam's former chief engineer

“I went to visit the site and saw what kind of stone there was there. A lot of gypsum and anhydrite, which are very soluble. I was really concerned; I told the director general. He said: ‘Don’t worry. This is all being taken care of.’”

In the preceding years, successive foreign consultants had pointed out the weaknesses in the rock formations but all assured the Iraqi government the problem could be solved by grouting. The decision to go ahead was pushed through by one of the regime’s vice-presidents, Taha Yassin Ramadan.

“Ramadan was very keen to have the dam,” Ansari said. “He wanted to show Saddam he was doing something brilliant, and he came from Mosul, so he wanted to do something that brought jobs to Mosul. This sped up the decision.”

The dam was designed by a Swiss firm of consultants and built by a German-Italian consortium in 1984. Water began seeping through in 1986, when it became apparent that the geological issues were worse than the consultants had predicted. From then on it required constant maintenance to fill the caverns being hollowed out by water running through the soluble bedrock. A total of 95,000 tonnes of grout of different types were used over the dam’s lifetime.

“All you are doing with grouting is prolonging the life of the dam. There is no permanent solution except building another dam,” Ansari said. A second structure, the Badush dam, was started 20km downstream, to prevent a catastrophe in the event of the Mosul dam’s failure. But work on Badush halted in the 1990s because of the pressure of sanctions, leaving it only 40% complete.

An international conference has been announced in Rome in April to discuss ways of preventing a disaster, but by then it could already be too late.

“Nobody knows when it will fail,” Adamo said. “It could be a year from now. It could be tomorrow.”