A large-scale oil spill, believed to be from a tanker struck by the US military, doubled in size between Wednesday and Thursday, endangering two protected areas off Oman and Iran, said Nina Noelle of Greenpeace Germany.

The tanker RIESCO was one of four attacked Tuesday in the Gulf of Oman. A fifth was destroyed in the Strait of Hormuz off Iran’s Kharg island.

The visible area of pollution measured about 400 square kilometers (154 square miles) on Thursday, Noelle said. Satellite images reviewed by The Associated Press showed the spill stretching almost 70 kilometers (43 miles) long.

The spill is located between the Musandam protected area on the Omani side, which includes one of the region’s most important seabird breeding islands.

On the Iranian side is the Hara-e Roud-e Gaz Protected Area in Hormozgan province that includes mangrove forests.

“The current situation in the Strait of Hormuz, together with our experience of oil spills since the beginning of the war, unfortunately suggests that an effective response is unlikely,” Noelle said.