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Six Greenpeace activists boarded a Shell oil drilling ship Monday in the middle of the Pacific Ocean and vowed to stay there to protest the company's plans to drill above the Arctic Circle, the environmental organization said.

Royal Dutch Shell confirmed that the protesters "illegally boarded the Polar Pioneer" about 750 miles northwest of Hawaii on Monday morning. It called the protest a "stunt" that jeopardizes both the crew and the protesters themselves.

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Cassady Sharp, a spokeswoman for Greenpeace, told NBC station KTUU of Anchorage, Alaska, that the protesters had enough food and other supplies to remain atop the Polar Pioneer until it reached the Port of Seattle, which has signed a lease to host Shell's Arctic drilling fleet.

"They're going to stay up there as long as they can," Sharp told the station. "They're not trying to interfere in the drilling. They're just trying to make their presence known."

Greenpeace said the protesters hope to fly a giant flag including the names of "millions of people from around the world who are opposed to Arctic oil drilling."

The Obama administration last week upheld the 2008 lease of Arctic drilling rights to Shell, and the Interior Department is expected to approve the company's drilling plan by the end of April.

Greenpeace protesters board the Royal Dutch Shell drilling vessel Polar Pioneer on Monday in the Pacific Ocean. Greenpeace

Greenpeace protesters celebrate successfully boarding a Royal Dutch Shell drilling ship Monday in the Pacific Ocean. Greenpeace

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