Oman is facing an impending environmental catastrophe as an alleged Russian shadow fleet tanker leaks oil across hundreds of square kilometres near a nature reserve, NGOs warn.

The ship was rocked by explosions, a maritime security firm told AFP, but the cause of the blasts is unclear.

Russia has built up a ghost fleet of old ships with opaque ownership to get around Western sanctions since its 2022 Ukraine invasion, and experts have said the vessels' poor condition increases the risk of oil spills.

Satellite imagery verified by AFP showed what appeared to be an oil slick spreading from a ship that has been stranded for over a month near al-Qibliyyah island, off Oman's southern coast.

It is far from the Strait of Hormuz, which has seen regular tanker attacks in the Middle East war.

"With oil apparently continuing to leak and the vessel stranded in a difficult location, the risk of an even larger environmental disaster remains serious," Nina Noelle of Greenpeace told AFP.

She warned of a "severe and long-lasting" consequence.

Greenpeace said the oil sheen had expanded from up to 20 square kilometres on July 24 to an estimated 600 on August 4, according to satellite imagery it reviewed. (Reporting by AFP)