The United Nations said an abandoned oil tanker moored off the coast of Yemen loaded with more than 1 million barrels of crude oil is at risk of rupture or exploding, causing massive environmental damage to Red Sea marine life, desalination factories and international shipping routes.

Meanwhile, Houthi rebels who control the area where the ship is moored have denied U.N. inspectors access to the vessel. Internal documents obtained by The Associated Press shows that seawater has entered the engine compartment of the tanker, which hasn’t been maintained for over five years, causing damage to the pipelines and increasing the risk of sinking. Rust has covered parts of the tanker and the inert gas that prevents the tanks from gathering inflammable gases, has leaked out. Experts say maintenance is no longer possible because the damage to the ship is irreversible.

For years, the U.N. has been trying to send inspectors to assess the damage aboard the vessel known as the FSO Safer and look for ways to secure the tanker by unloading the oil and pulling the ship to safety.

But one European diplomat, a Yemeni government official and the tanker’s company owner said that Houthi rebels have resisted. The diplomat said the rebels are treating the vessel as a “deterrent like having a nuclear weapon.” All three individuals spoke on condition of anonymity to discuss the subject with a reporter.

“They do say that openly to the U.N., ‘We like to have this as something to hold against the international community if attacked,’” the diplomat said. “Houthis are definitely responsible for failure of the U.N. to look at the ship.”

Money is also an issue, the diplomat said, adding that the Houthis initially were demanding millions of dollars in return for the oil stored in the tanker. The U.N. is trying to reach an arrangement where money could be used to pay workers and employees at Yemen’s Red Sea ports, the diplomat added.

Some experts, however, criticize both the Houthis and the U.N. for failing to fully understand the magnitude of the crisis with the abandoned ship.

Ian Ralby, founder of I.R. Consilium, who specializes in maritime and resource security, told the AP that U.N.’s efforts to send a team to assess the ship is “futile.” What the vessel needs is a salvage team, he said.

“It’s real shame that they wasted so much money and time in this futile operation,” said Ralby. “If you are taking these years to get a simple team to assess, we will not have a second chance to salvage,” he added.

Ralby, who has written extensively about the tanker, told the AP that amid declining oil prices the cost spent on cleaning up the environmental damage from an explosion or leakage will be much more than the millions worth of oil on the ship.

But the Houthis have refused to back down from their demands.

Mohammed Ali al-Houthi, the rebel group’s leader, blamed the U.S. and Saudis for not letting the rebels sell the oil, saying in a June 18 Twitter post that any “disastrous consequences ... God forbid,” that could result from the collapse of the vessel will be the responsibility of these two countries.

The Iranian-backed Houthi rebels are in control of the western Red Sea ports, including Ras Issa, 6 kilometers (3.7 miles) from where the FSO Safer tanker has been moored since the 1980s. They are at war with the internationally recognized government, which is backed by a Saudi-led coalition and the United States. President Abed Rabbu Mansour Hadi is in exile in Saudi Arabia and his government in disarray.

The floating tanker is a Japanese-made vessel built in the 1970s and sold to the Yemeni government in 1980s to store for export up to 3 million barrels pumped from oil fields in Marib, a province in eastern Yemen. The ship is 360 meters (1,181 feet) long with 34 storage tanks.

A senior official at the state-owned oil company in charge of the tanker, said because of a shrinking operational budget, which used to be around 20 million dollars a year before the war, the company could no longer afford to purchase fuel needed to run the boilers on the ship. The boilers are needed to power generators that, among other things, keep an inert gas that prevents explosions flowing. The tanker needs 11,000 tons of the fuel, which cost about 8 million dollars each year.

“After the stoppage of the boilers the strong majority of the equipment and the machines of the tanker stopped because they all depend on steam power,” the company official said. That includes the machines that power the ventilation system, which reduces humidity and prevents corrosion, he said.

Since 2015, annual maintenance on the ship has come to a complete halt and most crew members, except for 10 people, were pulled off the vessel after the Saudi-led coalition imposed a land, sea, and air embargo before waging an extensive air campaign to dislodge the Houthi rebels from areas they seized including the capital, Sanaa.