Tracking data show the Caroline Bezengi was carrying nearly 1 million barrels of Russian crude. Sources told Reuters the ship first reported "difficulties" near the coast of Yemen on Aug. 8.
No one has claimed responsibility for an attack on the vessel.
Oman, in its first public environmental report on the incident, said it was working to contain the leak near the Hallaniyat Islands.
Tracking records show the tanker was loaded at the Russian Black Sea port of Novorossiysk in April and transited the Suez Canal in late May.
Built in 2001, the Caroline Bezengi is part of a so-called shadow fleet of older tankers used by Russia. Those vessels often lack proper insurance and sail under flags of convenience to obscure ownership. Public shipping databases listed the Caroline Bezengi as flying Cameroon's flag, but it was among 39 vessels removed from Cameroon's registry in June.
The ship is under sanctions by the European Union, the UK,
Canada, and Switzerland.