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The United Nations is launching a crowd-funding campaign for an operation intended to prevent an ageing Yemeni oil tanker from unleashing a potentially catastrophic spill in the Red Sea, a senior official said Monday.

"We hope to raise $5 million by the end of June," David Gressly, the UN's humanitarian coordinator for the war-hit country, told an online press briefing, adding it was an "ambitious" target. 

"Today I launched a @UN crowdfunding campaign because we urgently need funds to start the emergency operation before it is too late," he said in a subsequent Twitter post.

The decaying 45-year-old oil tanker FSO Safer, long used as a floating storage platform and now abandoned off the rebel-held Yemeni port of Hodeida, has not been serviced since Yemen was plunged into civil war more than seven years ago. 

It is in "imminent" danger of breaking up, the United Nations warned last month.

An operation to transfer its 1.1 million barrels of oil to a different vessel could begin next month, according to a website for the crowd-funding campaign, which will begin accepting donations Tuesday. 

A UN pledging conference last month for the oil-transfer operation fell far short of its $80 million target, bringing in just $33 million. 

On Sunday, neighbouring Saudi Arabia said it would contribute $10 million.

A Saudi-led military coalition intervened in Yemen in 2015 after Houthi rebels seized the capital Sanaa the previous year.

Environmentalists warn the cost of the salvage operation is a pittance compared to the estimated $20 billion it would cost to clean up a spill. 

The Safer contains four times the amount of oil that was spilled by the 1989 Exxon Valdez disaster, one of the world's worst ecological catastrophes, according to the UN. 

The UN has said an oil spill could destroy ecosystems, shut down the fishing industry and close the lifeline Hodeida port for six months. 

It has said the operation needs to be completed by the end of September to avoid "turbulent winds" that pick up later in the year. 

The war in Yemen has killed hundreds of thousands of people and left millions on the brink of famine. 

But fighting has reduced since April when a truce went into effect, with the truce currently due to last until August.