MUSCAT (AFP) — Oman on Monday said it was working to contain an oil spill off its southern coast after a tanker ran aground in June inside a protected marine area, triggering calls for action by environmentalists.

It follows an investigation by Agence France-Presse (AFP) that found the ship had been stranded for weeks near al-Qibliyyah island after it was rocked by explosions.

The Oman Environment Authority on Monday said the ongoing spill had affected roughly 390 square kilometers, according to its most recent analysis.

The slick was concentrated around Oman’s Hallaniyat archipelago off the country’s southern coast, where a marine reserve was established last year to protect sensitive ecosystems home to diverse marine life, including rare sea turtles.

The country’s environment authority said rapid action was needed to protect the area’s biodiversity, including “coral reefs, turtle nesting areas, marine habitats and coastal areas of ecological importance.”

A report in the state-backed Oman News Agency quoted an official with the transport ministry as saying authorities were conducting aerial surveillance and had deployed divers.

The official said authorities were in the early stages of developing a plan to transfer the vessel’s oil cargo “to reduce the risks arising from the incident, ensure navigational safety and protect the marine environment.”

The UK maritime security company Ambrey is to take part in the salvage operation, a spokesperson for the firm told AFP.

“This is a highly complex and fast-moving situation,” the spokesperson said adding Ambrey was working with “several key stakeholders, including a leading oil spill response company.”

Omani authorities have not yet identified the cause of the spill. But British maritime security company Vanguard told AFP last week that the tanker was rocked by explosions in early June as it sailed off the coast of Yemen, causing water to enter several sections of the vessel.

Advocacy groups Greenpeace and PAX told AFP that the spill was on the verge of causing an “environmental disaster,” saying the tanker was carrying around one million barrels of crude.

The ship is believed to be part of Russia’s “shadow fleet,” which Moscow uses to circumvent Western sanctions on its oil exports.

The fleet includes ageing vessels that have faced criticism over maintenance, insurance and safety standards.

Tankers have been frequently targeted in the Gulf region over recent months as the US exchanged fire with Iran and its allied proxies.