An "oil spot" has been seen near the floating storage and offloading vessel abandoned offshore Yemen and tabbed by environmentalists as a floating bomb.

Saudi Arabia reportedly brought the spot — 50km west of the 407,000-dwt Safer (built 1976) — to the attention of the UN Security Council in a letter on Wednesday, Reuters reported.

The ship has been stranded for five years and carries more than 1m barrels of oil.