Saudi Arabia on Sunday pledged $10 million to help prevent an ageing Yemeni oil tanker from unleashing a potentially catastrophic spill in the Red Sea bordering its waters.

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.

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.

AFP