A rusting oil-storage vessel moored off Yemen’s Red Sea coast could rupture or explode, Western officials said Friday, warning of an environmental and humanitarian catastrophe if it breaks apart.

The FSO Safer—which holds more than 1.1 million barrels of oil, four times the 1989 spill from the Exxon Valdez in Alaska—is located near the port of Hodeidah, a key entry point for humanitarian aid in a country caught up in a six-year civil war. The ship is rapidly decaying after being abandoned five years ago when Iran-backed Houthi rebels took control of the nearby coastline.

The United Nations and foreign powers are pushing for access to the vessel, but the Houthis have since 2019 stonewalled requests, raising fears that the rebels are seeking to use it as leverage in peace negotiations.

Pipes and valves on the four-decade-old vessel have been damaged by corrosion and its cooling system has been taking on seawater in recent months, Western officials said.

“Pretty much any spark could set fire to the oil,” a U.N. official with knowledge of the Safer said.

A spill could contaminate most of Yemen’s western coast, with some oil passing into the Gulf of Aden, according to a report prepared for the British government and seen by the The Wall Street Journal. Recovery, including of local fishing stock, would take up to 25 years and $20 billion, the report said. An explosion would affect one of the world’s busiest and narrowest shipping lanes and temporarily close the Hodeidah port, according to Western officials.

The U.N. wants to assess the vessel before deciding on the scale of repairs and whether the oil—worth about $40 million if its quality hasn’t deteriorated—needs to be extracted. The Houthis have said they want the assessment and repair done in one visit, which critics say is aimed at ensuring the ship stays in place.

Deep mistrust on every issue has scuttled international peace efforts in Yemen. After the Houthis seized the capital, San’a, in late 2014, a Saudi-led coalition backed by the U.S. has tried to reinstall the government. Ground battles, airstrikes and a blockade against the Houthis have killed thousands of civilians, causing what the U.N. calls the world’s worst humanitarian crisis.

Yemen’s internationally-recognized government and the rebels agreed to a cease-fire in Sweden in 2018, but peace talks have stalled as both sides regularly violate the truce and have delayed agreed-upon prisoner swaps.

Western officials suspect the Houthis want to keep the vessel as a possible floating bomb to deter coalition attempts to retake Hodeidah. The Houthis deny the allegation.

“What kind of a line of defense would it make?” Houthi Foreign Minister Hisham Sharaf said in an interview. “We would of course not explode that thing to harm our people.”

Mr. Sharaf, however, conceded that the Houthis don’t trust the coalition not to use the situation to attack Hodeidah. “We will not allow any kind of tricks and moves that might try to use an environmental-cleaning operation for military purposes.”

Amid gridlocked attempts to broker peace in 2018, the Saudi-led coalition launched a large-scale offensive to try to recapture Hodeidah during which dozens of civilians were killed in air raids.

The U.N. has received permission to access the Safer before, and last year deployed a team to nearby Djibouti, but the Houthis revoked permission a day before departure. Western officials say they are more hopeful this time, given the vessel’s fast-deteriorating condition.

“We have been making progress over the last few weeks…we hope the necessary permits will come through soon,” said John Ratcliffe, humanitarian affairs officer with the U.N. Office for the Coordination of Humanitarian Affairs.

The 1,200-foot Safer is a Japanese-made vessel that was launched in 1976 before being sold to the Yemeni government in the 1980s to store oil for export.

Abdulwahed al-Obaly, an activist and former employee at the Safer, said the risk of a fire had increased dramatically after equipment meant to inject nitrogen into the tanker to prevent contact between oxygen and the crude broke. He called on foreign powers to immediately tow the tanker to a dry port to extract the oil.

Fears were compounded by the devastating August explosion in Lebanon’s capital, where tons of poorly stored chemicals seized years earlier from an unseaworthy ship detonated, killing hundreds and ruining swaths of Beirut.

For now, Western officials are trying to keep the Safer—which was a key issue at a U.N. Security Council briefing in July—separate from broader peace efforts.

“That said,” Mr. Ratcliffe said, “a major spill could easily overwhelm everything else—including wider political efforts to end the conflict.”