PRESIDENT Vladimir Putin says 30 Greenpeace activists arrested by Russia were not "pirates" but had broken the law in a protest against Arctic oil exploration, as the authorities detained all the campaigners pending trial.

On Tuesday, Russia opened a criminal probe into suspected piracy by the four Russian and 26 foreign Greenpeace activists, with charges carrying the maximum punishment of 15 years in prison.

Russian authorities placed the 30 activists in detention centres in and around the far northern city of Murmansk on Wednesday after they were moved ashore from the group's Dutch-flagged vessel following their protest in the Barents Sea earlier this month.

"I do not know the details of what has happened but it's completely obvious that of course they are not pirates," Putin told an international Arctic forum in the far northern city of Salekhard.

In his first comments on the high-profile seizure of the Greenpeace vessel, he said it was "completely obvious these people violated the norms of international law."

Putin's comments indicate the charges of piracy could be dropped during the investigation. A spokesman for the Investigative Committee also said earlier Wednesday the current charges might be changed if new evidence emerges.

Vladimir Markin, the spokesman for the Russian equivalent of the FBI, said in a statement that all the 30 activists had been detained "as suspects."

Greenpeace had been trying to draw global attention to the dangers of Russian-led efforts to develop the Arctic as ice breaks up due to global warming.

The group sent a team of inflatable boats to a platform of Russian state-controlled energy giant Gazprom in the Barents Sea earlier this month from the Arctic Sunrise.

The icebreaker was last week seized by the Russian authorities and towed to Murmansk.

The foreign activists are nationals of Argentina, Australia, Brazil, Canada, Denmark, the United States, Britain, the Netherlands, New Zealand, Ukraine, France, Italy, Turkey, Finland, Switzerland, Poland and Sweden.

A representative of the regional investigators in Murmansk told AFP the high-profile case was overseen by Moscow-based colleagues.

"That means it is all very serious," she said on condition of anonymity.

The environmentalists' detention drew condemnation from Greenpeace and generated concerns in the West.