Rights groups on Monday urged governments worldwide to donate generously to a UN fundraising campaign to rescue an oil tanker in Yemen.

Organizations including Amnesty International, Human Rights Watch, Mwatana for Human Rights, and International Crisis Group wrote a joint letter calling on the US, the UK, the EU, and others to swiftly allocate funds to the UN emergency operations to prevent the tanker from triggering a major environmental disaster.

With its load of more than 1 million barrels of crude oil, the tanker has been stranded off Yemen’s western city of Hodeidah since 2015 after the Iran-backed Houthis seized control of the city.

The Houthi occupation of the city prompted international engineers into fleeing the country, depriving the tanker of important maintenance.

The vessel attracted international attention three years ago after rust eroded its walls, sparking warnings of a major ecological disaster in the Red Sea in case of a leak or the tanker exploding.

“We, the undersigned organizations, urge you, as key donor governments, to immediately support the salvage operation that would prevent the supertanker from spilling hundreds of thousands of barrels of oil into the Red Sea,” they said in their letter. “We call upon you to honor your prior funding commitments and increase those commitments as much as is necessary to begin the salvage operation immediately.”

They criticized governments and donors for their slow response to the UN’s appeals for donations.

“We are deeply concerned about the lack of urgency and slow pace of donations from the international community that has brought Yemen perilously close to a new humanitarian and environmental disaster,” the letter said.

Given the Safer tanker’s large load, international experts predict it could spark an environmental catastrophe larger than the oil spills from the Exxon Valdez supertanker in the US in 1989.

The UN said the emergency operation to rescue the tanker was delayed due to a lack of funds, prompting it to launch an online crowdfunding campaign to bridge the gap in funds needed to save the tanker.

“It’s incomprehensible that the UN is now reduced to crowdfunding $20 million when the potential damages could be a thousand times greater. Donors should immediately step up to address this looming risk,” Michael Page, deputy Middle East and North Africa director at Human Rights Watch, said in a statement.

In March, the Houthis, who have long resisted calls for allowing international experts to inspect the decaying tanker, signed a memorandum of understanding with the UN that would allow the international body to transfer the tanker’s cargo to another ship.

Yemeni government officials believe that the Houthis agreed to sign the MOU after the UN promised to replace the old tanker with a new one.

In June, Houthi leader Hussein Al-Ezzi said he discussed with Peter-Derrek Hof, the Dutch ambassador to Yemen, the UN operational plan which includes replacing the rotten tanker with “a new and similar tanker.”