The United Nations said Wednesday it has raised the $75 million necessary to salvage a stricken tanker off Yemen, an emergency operation aimed at averting a disastrous Red Sea oil spill -- and a potential $20 billion cleanup.

The decaying 45-year-old FSO Safer, long used as a floating storage platform and now abandoned off the rebel-held Yemeni port of Hodeida, has not been serviced since Yemen plunged into civil war more than seven years ago.

UN officials last month warned that the ship -- which contains four times the amount of oil spilled in the Exxon Valdez disaster in 1989 -- was a ticking environmental time bomb requiring immediate action.

"We are able to announce we have now pledges and commitment sufficient to start the FSO Safer salvage operation," said David Gressly, the UN resident and humanitarian coordinator in Yemen and leader of the global body's efforts on the Safer.

"It's a very key milestone," he said, adding that donor pledges have now topped $77 million.

Yemen is suffering one of the world's worst humanitarian crises due to the war between the government and Huthi rebels who control the port of Hodeida.

The ship in question contains 1.1 million barrels of oil. The United Nations has said a spill could destroy ecosystems, shut down the fishing industry and close the lifeline Hodeida port for six months.

The result would potentially be the fifth largest oil spill from a tanker in history, with the clean-up costs alone reaching $20 billion.

The first phase of the salvage operation would stabilize the FSO Safer and transfer the oil to another vessel.

A second phase involving long-term storage of the cargo is estimated to cost another $38 million.

"We believe that we could meet that in a timely fashion," Gressly said of the cost.