BEIRUT — 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 Wednesday.

Crude oil has been leaking from the tanker Caroline Bezengi — a sanctioned tanker believed to be part of Russia’s “shadow fleet” — that was carrying nearly 1 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 an oil spill has been spreading quickly in recent weeks from the partially submerged tanker off the shores of Qabiliyah Island, endangering a marine protected area there.

Oman’s Environment Authority said it has been monitoring the leak and found that contamination has also reached the beaches of Ras Madrakah on mainland Oman, some 200 kilometers (125 miles) to the north of Qabiliyah Island.

The authorities also warned that the southern coasts of Masirah Island, over 300 kilometers (185 miles) north of the submerged tanker’s location, may also be affected in the coming hours.

The 247-meter (899-foot) Caroline Bezengi is believed to be part of Russia’s “shadow fleet” and has been sanctioned by the British government and the European Union for carrying Russian oil. It had left the Black Sea port of Novorossiysk, in Russia, in May, according to geospatial data tracking company SynMax Maritime.

The vessel’s owners are believed to be Shanghai-based. Recent satellite images showed the spill is covering an area of over 600 square kilometers (231 square miles), spreading quickly amid strong winds and currents of the monsoon season.

Oman, a sultanate on the southeastern coast of the Arabian Peninsula, has been a mediator in the ongoing war in Iran, which the United States and Israel launched on Feb. 28.