Greenpeace is demanding the release of 30 activists including six Britons detained at sea following protests over oil drilling in the Arctic.
Armed Russian officials abseiled from helicopters on Thursday onto Greenpeace's vessel Arctic Sunrise, which was in the Arctic's Pechora Sea near to oil company Gazprom's rig to protest against its attempts to drill for oil in the region.
Greenpeace said the crew, which includes six UK nationals, remained in the custody of Russian officials on Friday and the environmental group had not had any contact with the ship since it was taken over. Two activists had already been arrested by the coastguard for a protest in which they scaled the Gazprom oil rig.
The Arctic Sunrise is understood to be several days from land but Greenpeace said it appeared to be heading west towards Russian territorial waters.
Greenpeace campaigners have protested outside Russian embassies around the world, including London, and a letter demanding the release of the activists and the Arctic Sunrise icebreaker has been handed in to Russian officials.
Ben Ayliffe, head of Greenpeace International's Arctic oil campaign, said: "The safety of our activists remains our top priority and we are working hard to establish what is facing them. They have done nothing to warrant this level of aggression and have been entirely peaceful throughout.
"In our last phone call with the ship, the crew said that their spirits remains high and they have been boosted by messages of support from thousands of people who stand with them to oppose dangerous Arctic oil drilling."
Greenpeace claims the Arctic Sunrise was boarded illegally as the ship was inside Russia's exclusive economic zone (EEZ) in which foreign vessels should be free to navigate.
The environmental group also responded to claims that a safety pod used in the protest resembled a bomb, saying it was designed to keep activists warm, was around 3m by 2m (10ft by 7ft), brightly coloured and covered in Greenpeace logos.
The green group is campaigning against attempts by oil companies to extract oil from the waters of the Arctic, warning a spill would be highly environmentally damaging and that extraction of more fossil fuels will add to climate change.The Foreign Office said it had approached the Russian authorities and asked for consular access.