Greenpeace activists have boarded an Arctic-bound Shell oil rig in the middle of the Pacific Ocean, the environmental group and oil giant said Monday.

Shell denounced the action as a "stunt" and said it would not deflect it from its Arctic oil exploration plans.

The six activists planned to camp on the 38,000-tonne Polar Pioneer platform, which they boarded 750 miles northwest of Hawaii using inflatable boats from the Greenpeace vessel "Esperanza."

The six -- from the United States, Germany, New Zealand, Australia, Sweden and Austria -- have supplies for several days and can communicate with the outside world, Greenpeace said.

"We made it! We're on Shell's platform. And we're not alone. Everyone can help turn this into a platform for people power!" tweeted one of the six, Aliyah Field.

Johno Smith from New Zealand added: "We're here to highlight that in less than 100 days Shell is going to the Arctic to drill for oil.

"This pristine environment needs protecting for future generations and all life that will call it home. But instead Shell's actions are exploiting the melting ice to increase a man-made disaster."

A Shell spokeswoman, Kelly op de Weegh, blasted the action.

"We can confirm that protestors from Greenpeace have illegally boarded the 'Polar Pioneer,' under contract to Shell, jeopardizing not only the safety of the crew on board, but the protestors themselves," she said.

She added: "Shell has met with organizations and individuals who oppose energy exploration offshore Alaska. We respect their views and value the dialogue.

"We will not, however, condone the illegal tactics employed by Greenpeace. Nor will we allow these stunts to distract from preparations underway to execute a safe and responsible exploration program," she said in a statement.