MOSCOW (AFP) - Russian authorities on Wednesday put 30 arrested activists from environmental group Greenpeace in pre-trial jails after questioning several campaigners over a protest against Arctic oil exploration, the group said.

On Tuesday, Russia opened a criminal probe into suspected piracy by four Russian and 26 foreign Greenpeace activists who could face up to 15 years in jail if the case comes to trial.

They had been on board the group's Arctic Sunrise icebreaker, which the Russian security service seized last week and towed to the far northern port city of Murmansk with all 30 activists under arrest.

However they were taken ashore on Tuesday evening for questioning and 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.

The 30 activists have been taken to various detention centres in and around Murmansk after being questioned until early hours of Wednesday, she said.

A representative of the regional investigators in Murmansk confirmed that the activists had been questioned on Tuesday night.

The official requested anonymity as the high-profile case was overseen by Moscow-based colleagues.

"That means it is all very serious," she said.

Greenpeace confirmed its activists had been interrogated on Tuesday night after the team was first questioned by investigators aboard the Dutch-flagged vessel.

"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 said the activists were accompanied by Greenpeace lawyers.

"Diplomats had gathered outside with media and police, while some 15-16 Investigators had arrived earlier, together with translators," he added.

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.

The group had been trying to highlight the dangers of Russian-led efforts to develop the Arctic as ice floes break up due to global warming.

It sent a team of inflatable boats to the Gazprom platform in the Barents Sea earlier this month from the Arctic Sunrise icebreaker and 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 coast guard.

Agents from Russia's Federal Security Service (FSB) seized control of the activists' vessel the next day by descending onto the deck from helicopters in a commando-style raid.

The icebreaker is anchored off the coast of Murmansk after being towed from the scene of the incident by Russian border guards, in a voyage lasting several days.

The chief spokesman for the powerful Investigative Committee - Russia's equivalent to the FBI - said regional security authorities had launched a criminal probe for piracy "undertaken by an organised group".

Investigative Committee spokesman Vladimir Markin warned that the activists would be prosecuted regardless of their citizenship.

Greenpeace has condemned Russia's actions and said its supporters had already sent more than 415,000 emails and letters of support for the "Arctic 30" to Moscow's embassies around the world.

"Any charge of piracy against peaceful activists has no merit in international law," Greenpeace's international executive director Kumi Naidoo said in an emailed statement.

Russia's slice of the Arctic is generating growing interest from energy producers as gradually rising temperatures open sea lanes and start to reveal the vast oil and natural gas reserves thought to be buried below.

But Greenpeace argues that the firms have no plan in place to deal with potential oil spills in a previously unexplored environment that is home to polar bears, walruses and rare seabirds.

Greenpeace says the Russian action was illegal because the Arctic Sunrise was in international waters at the time of the raid.

But Mr Markin argued that the ship was "in the exclusive economic zone of the Russian Federation" when it was boarded by the FSB agents.