Move marks major step in preventing disastrous spill or explosion in key shipping lane after years of frantic warnings from governments and environmental groups

Arrival of Ndeavor, which set off from neighboring Djibouti, is the first step in a weeks-long effort to avert a major oil spill

UN appeals for $14 million needed to address the issue, says if the FSO Safer breaks up, it could unleash a catastrophic spill costing up to $20 billion to clean up

Iranian-backed Houthis have agreed to allow the transfer of the million-plus barrels of oil stored on ailing tanker FSO Safer, but emergency funds are still $20 million short

Environmentalists warn the cost of the operation is a pittance compared to the estimated $20 billion it would cost to clean up a spill

Two-month UN-brokered truce between Iran-backed Houthis and Saudi-led coalition is set to end this week; supertanker Safer stranded off Yemen's coast since 2015 poses major threat