The decaying 45-year-old oil tanker known as the 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.

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

The tanker, which lies some 150 kilometres (100 miles) south of the border with Saudi Arabia, is in "imminent" danger of breaking up, the United Nations warned last month.

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.

Last week environmental campaign group Greenpeace urged the Arab League to drum up funds for an operation that would transfer its 1.1 million barrels of oil to a different vessel.

A UN pledging conference last month fell far short of its $80 million target, bringing in just $33 million.

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

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. 

Riyadh will donate $10 million to the effort through the King Salman Humanitarian Aid and Relief Center, the official Saudi Press Agency reported Sunday. 

"The Kingdom calls on the United Nations to quickly take the necessary measures to ensure the prevention of oil leakage... and also calls on the international community to contribute urgently to support this initiative and prevent a serious environmental disaster," the agency said.

Saudi Arabia's current defence spending is $36.8 billion per year, according to the Military Balance+ database prepared by the International Institute for Strategic Studies.

Ghiwa Nakat, executive director at Greenpeace for the Middle East and North Africa, urged the Arab League's secretary-general "to hold an urgent meeting and make concerted efforts to fund the plan to rescue the Safer before it is too late and before disaster strikes."

Nakat said it was "deplorable that the Safer crisis has yet to be resolved due to the lack of financial support".

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.

"It is high time to resolve the Safer crisis and make every possible effort to avert the disaster looming on the horizon, particularly since this crisis will affect Arab states first and foremost," Nakat said.

"We trust that the (Arab League) is capable of playing this role and expediting the solution. If disaster strikes, its harsh consequences will affect us all, along with millions of people living in the region who will see their livelihoods, nutrition, health, and environment deteriorate."

Environmentalists warn the cost of the funds needed to carry out the operation is a mere pittance compared to the estimated $20 billion it would cost to clean up a spill in the pristine waters of the Red Sea.

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

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.