India, Aug. 13 -- Oil leaking from a tanker grounded off the coast of Oman is not only endangering a nearby island but has now also reached beaches on the Gulf Arab country's mainland, to the north of the island, Oman's environment authority said.

Crude oil has been leaking from the tanker Caroline Bezengi, a sanctioned tanker believed to be part of Russia's "shadow fleet", which was carrying nearly a million barrels of oil when it reported an explosion in June. No party has claimed responsibility for the incident and the cause of the explosion has not been revealed.

Environmental experts said the spill has been spreading quickly in recent weeks from the partially submerged tanker off the shores of Qabiliyah Island, endangering a marine...