The transfer of 1.1 million barrels of oil from an aging tanker moored off the coast of war-torn Yemen has been completed, avoiding an environmental disaster, the United Nations said.

An international team began siphoning the oil from the dilapidated vessel known as FSO Safer on July 25. All of the oil is now aboard a replacement tanker called the MOST Yemen.

Before the transfer, the Safer, which Yemen used as a floating storage and offloading facility, held four times as much oil than was spilled in the 1989 Exxon Valdez disaster off Alaska, one of the world’s worst ecological catastrophes, according to the UN.

For years, international organisations and rights groups have warned of a potential spill or even an explosion from the tanker, which has not been maintained for years and has seawater in its engine compartment and damaged pipes.

It is moored 6 kilometres (3.7 miles) from Yemen’s western Red Sea ports of Hodeida and Ras Issa, a strategic area controlled by the Iranian-backed Houthi rebels who are at war with the internationally recognised government.

Both warring sides blamed the other for blocking a salvage operation that would see the oil removed, until a UN-led initiative succeeded in accessing the ship and raising money from international donors.

The transfer marks a major milestone in a plan that needs additional funding to transport the oil away and to move the SOF Safer.

The UN said a small amount of oil remains inside the Safer’s hull and that the salvage team needs to install a secure system for mooring the replacement tanker in deep water.

A statement from the UN read: “As much of the 1.14 million barrels has been extracted as possible. However, less than 2% of the original oil cargo remains mixed in with sediment that will be removed during the final cleaning of the Safer.”

The United States welcomed the news of the operation’s success and called on other countries to contribute to see the job through to the end.

Anthony Blinken, the US secretary of state, said: “The UN urgently needs the international community and private sector’s financial support to fill the funding gap needed to finish the job and address all remaining environmental threats.”

The floating tanker is a Japanese-made vessel built in the 1970s and was sold to the Yemeni government in 1980s to store for export up to three million barrels pumped from oil fields in Marib, a province in eastern Yemen.

The ship is 360 meters (1,181 feet) long with 34 storage tanks.

Peter Berdowski, CEO of maritime services company Boskalis, said the Safer’s former cargo was now inside a “modern double-hulled tanker”.

The UN contracted a Boskalis subsidiary, SMIT Salvage, to remove the oil.

He congratulated the company’s salvage team for “carrying out the work under very challenging conditions in the Red Sea”.