UN Secretary General Antonio Guterres "welcomes the news that the ship-to-ship transfer of oil from the FSO Safer to the Yemen replacement vessel has been safely concluded today, avoiding what could have been a monumental environmental and humanitarian catastrophe," a statement said.
The milestone means that "the core aspect" of a years-long effort to address the threat posed by the Safer, often referred to as a "ticking time bomb", is now finished, said UN Development Programme head Achim Steiner.
"That removes the imminent and immediate threat that had become the focus of attention across the whole world: a tanker that could break apart or explode in the Red Sea," Steiner told AFP.
Yemeni doctor Main Ahmed, 49, told AFP Friday's news was like "a weight lifted off our shoulders" for residents of the port city of Hodeida.
"I was always thinking about the risks of just going for a walk, especially since I live near the coast," he said.
Yet the saga is not over.
The UN has previously warned that even with its cargo removed, the Safer "will pose a residual environmental threat, holding viscous oil residue and remaining at risk of breaking apart".
The project's next phase, stripping and cleaning the Safer's tanks and preparing it for towing and scrapping, is expected to take "anywhere between two to three weeks", Steiner said.
The Safer, a floating storage and offloading facility, has been moored around 50 kilometres (30 miles) from Hodeida since the 1980s.
It has not been serviced since war broke out more than eight years ago between Yemen's Huthi rebels, who control the capital Sanaa and the waters where the Safer is positioned, and a Saudi-led coalition backing the internationally recognised government based in the southern port city of Aden.
The ageing vessel, with its corroding hull, was carrying 1.14 million barrels of Marib light crude, four times as much oil as was spilled in the 1989 Exxon Valdez disaster off Alaska.
'Absence of accountability'
Now its oil has been pumped to a new, smaller tanker known as the Yemen that the UN purchased in March.
As the operation began on July 25, experts warned that success was far from certain given scorching summer temperatures, ageing pipes and naval mines lurking in surrounding waters.
The UN had even arranged for a plane to be on standby "within a 90-minute flight radius" so it could "deploy chemicals from the air" in response to a spill, Steiner said.
All told, the UN has priced the operation at $143 million, a fraction of the estimated $20 billion clean-up costs in the event of a spill, to say nothing of billions lost to shipping disruptions through the Bab al-Mandab strait to the Suez Canal.
The UN is still roughly $20 million short, and US Secretary of State Antony Blinken urged the international community and the private sector Friday to step up with funds needed to "finish the job and address all remaining environmental threats".
Greenpeace said oil giants that previously used the Safer "and are the likely owners of some of the transferred oil" needed to contribute to the project's next stages.
"It is imperative to address the absence of accountability exhibited by the oil industry that has recorded staggering profits, but hasn't yet shown any sense of responsibility," a Greenpeace statement said.
Next steps unclear
Officials are still trying to sort out where the oil might go next.
A long-term agreement between Yemen's warring parties on "future maintenance and management" is needed, Steiner said.
One possibility is a follow-up project in which UN experts would train staff of the Yemeni oil and gas company SEPOC in "how to safeguard the vessel", he said.
However authorities in Sanaa and Aden have appointed rival executive general managers of SEPOC, underscoring how Yemen's conflict could continue to pose complications.
Hundreds of thousands of people have died in the fighting or from indirect causes such as lack of food or water, in what the UN calls one of the world's worst humanitarian crises.
Clashes between the Iran-backed Huthis and the Saudi-led coalition have reduced sharply since a UN-brokered truce began in April last year, even though it lapsed in October.
David Gressly, the UN resident coordinator in Yemen, said on Friday that while the Safer crisis was separate from talks on ending the war, the fact that opposing factions cooperated on the oil transfer "does create a bit of hope that there is a way forward".