U.S. pollution-fighting experts arrived at the Galapagos Islands on Sunday to help clean up a spill from a stricken tanker that has so far dumped some 150,000 gallons of fuel oil into the fragile marine environment.

In the first account on the extent of the spill, Ecuadorean Environment Minister Rodolfo Rendon said the "environmental damage is extremely grave." He said surf pounding the Ecuadorean tanker Jessica, which ran aground Tuesday in a bay off San Cristobal Island, has opened up new fissures in its hull, speeding up the rate of the leak. So far, 150,000 gallons of fuel has leaked out and slicks have affected an 117-square-mile area. The ship, which began leaking fuel Friday, lay tilted sharply toward its left side about 550 yards off shore from San Cristobal, which is populated with rare marine species.

The Galapagos Islands, in the Pacific Ocean 600 miles off Ecuador's coast, are famous for their giant tortoises and rare species of birds and plants. Charles Darwin developed his theory of evolution by studying wildlife on the islands.

The team of U.S. experts, including 10 members of the U.S. Coast Guard's pollution response National Strike Force, arrived late Sunday, Rendon said. They reportedly arrived with specialized oil spill equipment, such as inflatable oil containment barges, and high-capacity pumps to help remove remaining fuel from the ship's storage tanks.

Floating nets and barriers have been set up to control the spill, but officials said slicks had reached nearby beaches and harmed at least 11 sea lions, which were expected to live. Some 20 birds, including blue-footed boobies, pelicans and albatrosses, also were affected.

Galapagos National Park biologist Mauricio Velasquez said the long-term danger is that the fuel will sink to the ocean floor, destroying algae that is vital to the food chain, threatening marine iguanas, sharks, birds and other species.

Velasquez said the current was pushing the spill south, and that within days it could reach Spain Island, where large colonies of sea lions and other marine animals congregate.

Also Sunday, Ecuadorean President Gustavo Noboa demanded a "detailed report" on the cause of the accident, which officials have attributed to navigational error.

Police on San Cristobal said no charges had been filed against the ship's captain, Tarquino Arevalo, who remained on the island Sunday, or against his company, Acotramar.