Armed Russian coastguards forcibly boarded Greenpeace’s Arctic Sunrise ship, according to the environmental group.
Russian authorities have detained the vessel and threatened to bring criminal charges after activists scaled the country’s first Arctic offshore oil platform.
Moscow has said the ship, registered in the Netherlands, is being towed to Murmansk with 27 people on board.
Japser Teulings from the Greepeace International General Counsel said: “We don’t know whether the ship has been seized and we would be surprised if it had been because that would have been illegal. Under international law a ship has free right of passage, it was in international waters, outside of territorial waters of the Russian government at the moment it was boarded.”
Russia has denied claims that the vessel was in international waters when it was seized and called the protest “aggressive and provocative”.
Greenpeace has said the real threat to the region was reckless energy exploitation.