The United Nations has purchased a large tanker to store about 1.1 million barrels of oil that will be transferred from a decaying vessel off Yemen's coast in a bid to avert an environmental disaster, officials said on Thursday.

UN officials have been warning for several years that the Red Sea and the coastline of Yemen was at risk as the Safer tanker could spill four times as much oil as the 1989 Exxon Valdez disaster off Alaska.

So far pledges of $95 million have been made, mostly by governments - of which $75 million has been paid. Even a public crowdfunding drive was started last year, which Gressly hopes can help provide more money to "finish the job."

Gressly, wanting to spur more public donations, praised students at an elementary school in Bethesda, Maryland, for raising $200 by selling lemonade.

Explosion risk

UN Development Program Administrator Achim Steiner described the price of the tanker as "painful" amid a hot market driven by factors stemming from the Ukraine conflict, but Gressly said the world body had no other choice. A year ago it could have cost $10-15 million less, he said.

"We hope, if all things go according to plan, that the operation of the ship-to-ship transfer would actually commence in early May," Steiner told reporters.

The operation cannot be paid for by the sale of the oil because it is not clear yet who actually owns it, the U.N. officials said.

The Safer supertanker was being used as a floating storage and offloading facility and is moored off Yemen's Red Sea oil terminal of Ras Issa. Production, offloading and maintenance operations were suspended in 2015 because of the war in Yemen.

The UN has warned that the tanker's structural integrity has significantly deteriorated and it is at risk of exploding.

"Work must now begin urgently. There is no time to delay," said Britain's UN Ambassador Barbara Woodward. Britain last year pledged some $7 million for the operation, she said.

Yemen has been mired in conflict since the Houthi rebel group ousted the government from the capital Sanaa in 2014. A Saudi-led military coalition in 2015 intervened in a bid to restore the government.