The United Nations warned Monday that the oil spill off Yemen coast will have devastating consequences and it would cost $20 billion to clean up.

"Our recent visit to (the FSO Safer) with technical experts indicates that the vessel is imminently going to break up," the U.N. humanitarian coordinator for Yemen, David Gressly, said ahead of a conference hosted by the U.N. and The Netherlands to raise funds for an emergency operation to prevent an oil spill.

The 45-year-old FSO Safer, long used as a floating oil storage platform with 1.1 million barrels of crude onboard, has been moored off the rebel-held Yemeni port of Hodeida since 2015 without being serviced.

"The impact of a spill will be catastrophic," Gressly continued at a briefing in Amman. "The effect on the environment would be tremendous ... our estimate is that $20 billion would be spent just to clean the oil spill."

The U.N. official had earlier announced on Twitter that the Netherlands would host a pledging conference for the international body's plan to avert the crisis on Wednesday.

Last month, the U.N. said it sought nearly $80 million for its operation. It warned of "a humanitarian and ecological catastrophe centered on a country already decimated by more than seven years of war."

It said that the emergency part of a two-stage operation would see the toxic cargo pumped from the storage platform to a temporary replacement vessel at the cost of $79.6 million.

Gressly estimated that a total of $144 million would be needed for the full operation, reiterating that $80 million was needed "to secure the oil safely in the initial phase."

Hundreds of thousands of people have been killed directly or indirectly in Yemen's seven-year war, while millions have been displaced in what the U.N. calls the world's biggest humanitarian crisis.