The supertanker, with more than a million barrels of oil, has been deteriorating since 2015, leading to fears of a catastrophic spill.

The FSO Safer – a floating storage and offloading (FSO) tanker holding more than 1.14 million barrels of oil – has been at risk of breaking up or exploding for years due to its corrosion and a lack of maintenance since a Saudi-led coalition intervened in Yemen’s war in 2015.

Yemen’s Houthi rebels, who control the area of Yemen’s coast where the Safer is moored, had previously prevented any salvage operation from taking place, but finally agreed in March to allow for the oil to be offloaded.

The oil abroad the Safer will now be transferred into a replacement vessel, called Yemen, in a ship-to-ship transfer that is expected to last 19 days, said the UN Development Programme (UNDP) – the humanitarian agency in charge of implementing the operation.

“In the absence of anyone else willing or able to perform this task, the United Nations stepped up and assumed the risk to conduct this very delicate operation,” said UN Secretary-General Antonio Guterres on Tuesday.

“The ship-to-ship transfer of oil which has started today is the critical next step in avoiding an environmental and humanitarian catastrophe on a colossal scale,” he added.

The operation, the first of its kind, is a risky one – but the potential leaking of the remaining oil in the deteriorating tanker that the Yemeni government purchased in the 1980s is even more so.

Observers have worried for years that the Safer could crack or explode. The ensuing oil spill would have the potential to wipe out one of the world’s largest marine ecosystems.

Once the oil is offloaded, the delivery and instalment of a catenary anchor leg mooring (CALM) buoy will take place, the UN said.

The buoy will then be secured to the seabed, which in turn will be used to secure the replacement vessel, a process that must be completed by September, the international body noted.

A technical support vessel from the Dutch-based company Boskalis/SMITis would be ready to step in should any oil leaks occur during the operation.

The 47-year-old supertanker was left abandoned and has been out of service since the civil war broke out in Yemen eight years ago.

The Safer is anchored near the Ras Isa oil terminal controlled by Yemen’s Houthi movement, which in 2015 seized large parts of the country.

Mohammed Mudawi from UNDP Yemen told Al Jazeera in mid-July that the vessel had not been properly maintained as it was situated in an area littered with mines. The UN agency team had also been working on preventing the build-up of flammable gases.

“We have many concerns that it can explode because of the gases,” Mudawi said.

According to the UN, a major spill would destroy coral, mangroves and other sea life; expose millions of people to highly polluted air; devastate fishing communities; force nearby ports to close; and disrupt shipping through the Suez Canal. The cost of the clean-up alone is estimated at $20bn.

A spill from the Safer could potentially have an impact greater than one of the largest oil spills in history, the 1989 Exxon Valdez disaster, since the Safer carries four times that amount of oil, according to the UN.