"It was repeated several times in open conversation that they were threatening that if we didn't leave the Northern Sea Route that they would fire on our ship," Christy Ferguson, a Canadian crew member on the Arctic Sunrise steaming back to Norwegian waters, said Monday.
Russian officials weren't immediately available for comment.
The vessel entered Russian waters late Friday to witness and protest offshore energy exploration in that country's north.
Offshore Arctic exploration is controversial because there are no proven, reliable methods of cleaning oilspills from ice-choked, stormy seas. Seismic exploration by Russian companies and western multinationals is being conducted in areas that include marine national parks and other protected areas.
Russia requires a permit to enter the Northern Sea Route and Greenpeace applied for one three times. Permission was denied over concerns about the Arctic Sunrise's ice-worthiness, although the ship has a higher capability than dozens of other vessels that have been allowed in — including the six ships currently looking for oil and gas.
By Saturday, Ferguson said, the Arctic Sunrise had neared one of those ships operated by ExxonMobil and Russia's Rosneft. That night, a Russian coast guard ship appeared.
After questions about the icebreaker's operations and intent, the Russians asked permission to board. It was denied.
The coast guard ship remained nearby Sunday, but took no action. On Monday morning, the Greenpeace crew began their protest.
"We radioed over to that seismic ship and told them that we were coming closer. That we intended peaceful protest. That we would not interfere with their operations. We would not interrupt their navigation," Ferguson said.
The seismic ship didn't respond, but the coast guard again asked to board.
"They said dozens of times that they wanted to board our ship, that they wanted us to leave."
The Greenpeace crew went ahead and launched inflatable boats. At that point, crew members received another message.
"The coast guard told us that if we didn't let them board, they would use preventive fire — firing in front of the ship."
The inflatables were called back and the crew didn't prevent four unarmed Russian coast guard officers from boarding. They subjected the Arctic Sunrise to a lengthy inspection which, Ferguson said, found nothing wrong.
"They asked us a lot of questions about why we were here? What are our intentions? They were communicating back and forth to their captain and they told us that their captain demanded that we leave the Northern Sea Route area immediately and that if we didn't, he would use 'strict measures.'
"We asked, 'What do you mean by strict measures?' They confirmed that what they meant by strict measures was fire.
"Preventive fire, at first."
Ferguson, who was in the meeting between her ship's captain and the four officers, said the discussions were calm and reasonable. But the manner of the coast guard officers as they continued to receive instructions from their superiors convinced the Sunrise's captain that the threat was real.
"It was a tone and feeling thing, but at a certain point it seemed to both of us that this was a real possibility. It wasn't just a bluff.
"We decided the risk to our vessel and our crew was too high. The threat of force seemed actually credible. They might actually follow through on it.
"We agreed to leave."
Because of stormy weather, it will take the Arctic Sunrise four days to leave Russian waters. It was being shadowed Monday by the coast guard.
Despite not being able to observe the seismic activity and carry out their protest, Ferguson said the trip was revealing.
"We came here to expose preparations for oil drilling and what we ended up exposing was that, and something else — the efforts of the Russian authorities to defend oil interests."
The Arctic Sunrise is registered in the Netherlands and the Dutch government has expressed support for the vessel.
"The right of Greenpeace to peaceful protest is beyond dispute," the Dutch said in a translation provided by Greenpeace.
"We asked the Russian authorities on Friday what Greenpeace would need to do in order to meet Russian requirements at short notice and requested a prompt reply. So far we did not receive a reply."
Arctic international law expert Michael Byers said the legality of the Russian actions is murky. He said that while Greenpeace was operating outside territorial Russian waters, international law does give Arctic nations strong pollution prevention jurisdiction out to 200 nautical miles.
Both sides may be pleased with the outcome, he said in an email.
"Russia has reinforced its generous interpretation of coastal states rights. Greenpeace draws attention to Russia's rapid opening of its Arctic offshore to oil exploration which, given that country's poor record on oil leaks and spills, does not bode well for the marine environment."