For years, a rusty oil tanker has been moored off war-torn Yemen -- abandoned and threatening to break up or explode in what would be an ecological and humanitarian catastrophe.

On Tuesday the United Nations said it had reached an "agreement in principle" with Yemeni rebels to pump out the volatile cargo from the FSO Safer vessel and transfer it to another ship.

The 45-year-old ship, long used as a floating oil storage platform, has been moored off Yemen's western port of Hodeida in the Red Sea since 2015, without being serviced.

It is moored about 60 kilometres (40 miles) from the nearest inhabited areas with 1.1 million barrels of crude on board.

Apart from corrosion to the ageing hull, essential work on reducing explosive gases in the storage tanks has been neglected for years.

Greenpeace has warned the vessel could "explode at any moment".

Experts said the latest problem emerged in May 2021 with a leak in a cooling pipe, which was later fixed.

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.

Independent studies show it could expose more than 8.4 million Yemenis to heightened pollution.

Maritime traffic and coastal countries including Djibouti, Eritrea and Saudi Arabia could also be affected.

About 80 percent of Yemen's population depends on some form of aid for survival, with the civil war between the Saudi-backed government and Iran-allied Huthi rebels showing no signs of abating.

Repeated delays

Inspection of the deteriorating ship has dragged on for years with UN requests for access repeatedly declined by the Huthis, who control much of Yemen's north including Hodeida port.

The rebels have said they want guarantees that the value of the oil on board the Safer would be used to pay salaries of their employees.

The Yemeni government has said the money should be used for health and humanitarian projects.

In November 2020, the Huthis said they gave the green light for a mission to assess the fuel tanker.

The UN initially planned it in early 2021, but it has been repeatedly delayed.

The UN last year urged the Huthis to "facilitate unconditional and safe access for UN experts to conduct a comprehensive and impartial assessment and initial repair mission without further delay".

The UN said on Tuesday that an "agreement in principle" has been reached to transfer the toxic cargo from the tanker to another ship, without giving further details or a timeline.

"I am pleased to report recent progress in efforts to resolve the Safer tanker issue, including an agreement in principle to a UN-coordinated proposal to shift the oil to another ship," said Martin Griffiths, the UN's deputy chief for humanitarian affairs.

Yemen's civil war broke out in 2014 when the Huthis seized Sanaa, prompting a Saudi-led military coalition to intervene the following year to prop up the internationally recognised government.

Hundreds of thousands of people have been killed directly or indirectly in the conflict, while millions have been displaced in what the UN calls the world's biggest humanitarian crisis.