Six Greenpeace activists have boarded a Russian offshore oil rig in protest against gas and oil exploration in the Arctic, the group says.
The activists, who included the organisation's executive director Kumi Naidoo, reached the Prirazlomnaya platform by speedboat early on Friday.
"The only way to prevent a catastrophic oil spill from happening in this unique environment is to permanently ban all drilling now"
- Kumi Naidoo, Greenpeace global chief
They say they have enough supplies to last "several days".
The activists are protesting against Russia's drilling in the area, calling the plans "dangerous" and asking for it to be abandoned.
They say that an oil spill in the Arctic region could be "catastrophic" as the extreme conditions are likely to stall any emergency operations.
Three inflatable speedboats had approached the platform after 4am local time to let the six activists climb it via mooring lines using ropes and ladders, the group said.
The activists have taken portable tents, along with the supplies, said a witness aboard a Greenpeace ship about 6km away from the platform.
The Prirazlomnoye field, Russia's first Arctic offshore development, has been plagued by delays due to cost overruns and platform construction difficulties, with oil now expected to flow at the turn of the year.
The Russian energy giant Gazprom and Gazprom Neft Shelf, a subsidiary holding the Prirazlomnoye license, declined immediate comment.
'Ban all drilling'
Prirazlomnoye is estimated to hold reserves of 526 million barrels and success in launching Arctic oil exploration is seen as vital to sustaining Russia's long-term status as the world's second biggest oil producer.
Environment campaigners warn that remoteness, fragile ecosystems, darkness, sub-zero temperatures, ice, high winds, and other environmental conditions could stall emergency operations in the case of a spill.
"The only way to prevent a catastrophic oil spill from happening in this unique environment is to permanently ban all drilling now," said Naidoo.
Greenpeace will seek to promote a resolution at the UN General Assembly that would declare the Arctic part of the global commons, Naidoo said earlier this month in Moscow.
The global commons includes those parts of the Earth's surface beyond national jurisdictions.
Naidoo said that this would prevent any drilling there.