The United Nations has secured insurance coverage for a ship-to-ship transfer of 1.1 million barrels of crude oil from a rusting tanker moored off the coast of war-torn Yemen.

The United Nations Development Programme (UNDP) described the insurance as “a pivotal milestone” in a years-long effort to evacuate the cargo of the FSO Safer, which is at risk of rupture or exploding and causing an environmental catastrophe.

The UNDP has been trying to start a salvage operation to avert what it says could amount to “one of the world’s largest, man-made disasters in history”.

It secured tens of millions of dollars in pledges for the operation, which started late in May with experts pumping inert gas to remove atmospheric oxygen from the oil chambers of the vessel.

“Insurance became a critical element of enabling this salvage operation to proceed. Without it, the mission could not go forward,” said Achim Steiner, a UNDP administrator.

Transferring the stored oil is expected to start later this month, according to David Gressly, the UN humanitarian coordinator for Yemen. After completing the transfer of oil, Safer would eventually be towed away and scrapped, he said.

“Work is progressing well,” Mr Gressly told the Yemen International Forum at The Hague.

The tanker was built in Japan in 1980 and the Yemeni government purchased it in the 1980s to store up to 3 million barrels of oil pumped from fields in Marib, a province in the east of the Arabian Peninsula country.

Yemen, the Arab world’s most impoverished country, has been engulfed in civil war since 2014, when the Iran-backed Houthi rebels seized the capital, Sanaa, and much of the country’s north, forcing the government to flee to the south, then to Saudi Arabia.

The following year, a Saudi-led coalition entered the war to fight the Houthis and try to restore the internationally recognised government to power.

The Safer is 360 metres long with 34 storage tanks. It has not been maintained since 2015, and in recent years, seawater entered its engine compartment, causing damage to pipes and increasing the risk of sinking.

Rust has covered parts of the tanker and the inert gas that prevents the tanks from gathering inflammable gases has leaked out.