UNITED NATIONS (AP) — The United Nations said Tuesday it is “very concerned’’ that Yemen’s Houthi rebels may reconsider their formal approval for U.N. experts to examine an oil tanker moored off the war-torn country’s coast loaded with more than 1 million barrels of crude oil that is at risk of leaking.

The U.N. warned last year that the tanker, the FSO Safer, hasn’t been maintained for more than five years and could cause an environmental, economic and humanitarian catastrophe. Experts fear the tanker could explode or leak, causing huge environmental damage to marine life and affect shipping in the Red Sea.

U.N. spokesman Stephane Dujarric said the Iranian-backed Houthis have not responded to multiple requests for a letter with security assurances that the U.N. needs to facilitate the leasing of “technically equipped service vessels” that are required for the mission. Without a letter, he said, the cost of the mission would increase by “hundreds of thousands of dollars.”

“We are also very concerned by indications that the Houthi de-facto authorities are considering a `review’ of their formal approval of the mission to deploy,” Dujarric said. “Houthi officials have advised the U.N. to pause certain preparations pending the outcome of such process, which would create further delays to the mission.”

The Houthis accepted the scope of work for the technical mission in November, and at that time Dujarric said the U.N. thought the expert team could get to the tanker by early March.

However, in light of the new “challenges, the timeline of deployment of the mission remains uncertain and dependent on the continued facilitation of all stakeholders involved,” Dujarric said.

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The rebels control western Yemen’s Red Sea ports, including Ras Issa, located 6 kilometers (3.7 miles) from where the Japanese-built FSO Safer tanker has been moored since the 1980s, when it was sold to the Yemeni government. The Houthis are at war with the internationally recognized government, which is backed by a Saudi-led coalition and the United States.

Prior to the escalation of the conflict in 2015, the vessel was used to store and export oil from fields in Marib, a province in eastern Yemen.

Internal documents obtained by The Associated Press last June show that seawater has entered the engine compartment of the tanker, causing damage to pipes 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, according to an AP report.

Dujarric said that “the United Nations remains committed to doing everything possible to address the grave humanitarian and environmental threat posed by the Safer tanker.”

“We hope to receive a renewed commitment from the Houthi de-facto authorities to resolve this urgent matter as soon as possible,” he said. “Any other outcome would be extremely disappointing.”