A vast oil slick from the leaking tanker Caroline Bezengi has reached Oman’s coastline, raising fears of one of the worst oil spills in years.

Axar.az reports, citing Reuters, that Oman’s environment agency said the spill could affect about 40 km of coastline near Ras Madraka as well as Masirah Island.

The slick has grown to more than 2,000 square kilometers, according to oil spill specialist John Amos, who reviewed satellite imagery.

The tanker, carrying an estimated 800,000 barrels of Russian oil and under international sanctions, ran aground on June 30 and has continued to leak. It is located near an island that forms part of an Omani marine nature reserve home to Arabian Sea humpback whales and Socotra cormorants.

The International Oil Pollution Compensation Funds (IOPC Funds) told Reuters it would not cover cleanup costs because the incident was being treated as an “act of war.”

Oil spill specialist John Amos warned that the tanker could break apart and release its entire cargo.

“A nightmare scenario is there's no adequate response to keep the worst from happening.”

Amos said the spill could reach 40 million to 50 million gallons if the vessel continues to deteriorate.

“The longer we wait, the more damaged and decrepit that tanker gets, the more complicated and difficult and potentially dangerous that salvage operation is going to be.”

The International Maritime Organization said seasonal monsoon conditions had restricted access to the tanker and delayed salvage operations.

“Oil spill contingency planning is in place,” an IMO spokesperson said.

Greenpeace said the slick had expanded rapidly, growing from around 45 square kilometers at the end of July to 150 square kilometers at the beginning of August, before quadrupling within days.