President Vladimir Putin said Wednesday 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 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."

'It is all very serious'

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.

After being taken ashore Tuesday evening for questioning, the activists were then put in detention centres where suspects are held before trial, known in Russia as Investigative Isolators (SIZO).

"They have been transferred to pre-trial detention centres," said Yevgenia Belyakova, a Greenpeace activist, saying the questioning lasted until the early hours of Wednesday.

The environmentalists had been detained for 48 hours.

"The Greenpeace International activists and crew came off the ship at the end of the day and were taken by two buses to the offices of the Investigative Committee in Murmansk," another Greenpeace spokesman, Aaron Gray-Block, said in emailed comments.

"Only five crew were interviewed before a halt was called for the night. No formal charges have been laid yet," he added, saying the activists were accompanied by Greenpeace lawyers.

Markin said however that just three Russian activists had been interrogated. The foreign nationals will be questioned as soon as translators and lawyers could be provided for all of them, he added.

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.

'We did not know who they were'

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

Finland's president Sauli Niinistoe raised the issue in a meeting with Putin on Tuesday, Kremlin spokesman Dmitry Peskov said. Foreign diplomats from countries like Poland and Sweden said they were in touch with Russian authorities over the case.

But Putin sought to defend the Russian authorities' tactics, saying it was not immediately clear to Russian officials those storming the platform really were campaigners from Greenpeace.

"Our law enforcement agencies, our border guards did not know who was trying to seize the platform under the guise of Greenpeace," he told the Arctic forum.

Greenpeace released photos of the detained team being taken to the investigators' offices in Murmansk in aged clunky buses, with the smiling activists flashing victory signs through the window.

During the protest in the Barents Sea, Greenpeace hitched two activists to the side of the rig.

The pair tried to scale the platform but eventually slipped into the freezing water and were recovered by the Russian coastguard.

Russian security services seized control of the activists' vessel the next day by descending onto the deck from helicopters in a commando-style raid.

The high-profile arrests of the Greenpeace activists come as Putin's foreign and domestic policies grow ever more assertive.

Last week, Putin lambasted the West and pledged to fiercely protect Russia from foreign influence, saying Russia's sovereignty, independence and integrity were "red lines" that could not be crossed.