Two Kiwi activists have been arrested by Russian authorities who boarded a Greenpeace ship protesting against Gazprom's Arctic oil drilling operations.

Greenpeace said the names of the New Zealanders, who were among 25 activists arrested aboard the Arctic Sunrise, have not been released for legal reasons.

The Arctic Sunrise had been circling Russian oil giant Gazprom's Prirazlomnaya platform at the three nautical mile limit, inside international waters and within Russia's exclusive economic zone.

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Greenpeace New Zealand spokesman Phil Crawford said armed Russian Coastguard officials boarded the ship by helicopter and ropes before rounding up the crew on the deck.

Activists locked inside the radio room said they saw other activists detained on their knees with guns pointed at them, he said.

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Greenpeace International executive director Kumi Naidoo said the boarding was illegal, and showed the Russian government would go to extreme lengths to stop public scrutiny of Gazprom's Arctic drilling.

The Russian Coastguard on Wednesday arrested and held without charge two Greenpeace activists who had scaled Gazprom's drilling platform.

Russia's Ministry of International Affairs said the Coastguard intervened because the Arctic Sunrise was an environmental and security threat, which Greenpeace refutes.

- APNZ