U.N. officials have been warning for years that the Red Sea and Yemen's coastline was at risk as the Safer tanker could spill four times as much oil as the 1989 Exxon Valdez disaster off Alaska.
"The ship-to-ship transfer of oil which has started today is the critical next step in avoiding an environmental and humanitarian catastrophe on a colossal scale."
The oil transfer is expected to take 19 days to complete, the United Nations' Development Programme (UNDP) said in a statement.
"We are obviously very cautious – it's only the beginning of a transfer," UNDP spokesperson Sarah Bel told a Geneva press briefing when asked about the operation's risks.
"The cost of an oil spill is estimated to be approximately $20 billion, and it will take years to clean up," she added.
Persons:
Antonio Guterres, Sarah Bel, Nadine Awadalla, Nayera Abdallah, Emma Farge, Clauda Tanios, Louise Heavens, Jan Harvey
Organizations:
United Nations, Programme, UNDP, Thomson
Locations:
DUBAI, GENEVA, Alaska, Yemen, Geneva, Dubai