A Greenpeace ship seized by Russian security forces during an Arctic protest approached the Far Northern port city of Murmansk on Tuesday where its activists risk facing formal charges.

Thirty activists from the group -- including four Russians -- are on board the vessel. Russia's powerful Investigative Committee said the crew may have committed piracy, an offence that in Russia carries a prison term of up to 15 years.

The group's Russian Arctic programme coordinator Yevgenia Belyakova said that the Arctic Sunrise entered the waters of the Kola Gulf off Murmansk around 7:00 am (0300 GMT).

The ship "is opposite the city of Severomorsk, around 20 kilometres (12 miles) from Murmansk. The Arctic Sunrise is being pulled by a (Russian border guards) tugboat," Belyakova said in an e-mailed statement.

The environmental lobby group's Dutch-flagged icebreaker had been monitoring the exploration activities of the Russian energy giant Gazprom since August in the hopes of exposing the dangers of drilling for oil in one of the world's great nature reserves.

Gazprom argues that it is following international safety standards and that its initial seismic survey activities were being strictly monitored for their environmental impact.

Russia's slice of the Arctic is generating growing interest from energy producers as global warming breaks up ice flows and reveals the vast oil and gas reserves believed to be buried below.

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

The Arctic Sunrise was stormed by agents from the Federal Security Service -- Russia's chief successor to the Soviet-era KGB -- during a dramatic helicopter operation on Thursday.

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

"Our activists did nothing to warrant the reaction we've seen from the Russian authorities," Greenpeace's International Executive Director Kumi Naidoo tweeted.

Greenpeace said its lawyers were awaiting the ship's arrival near Murmansk and expecting to learn from the Russians what charges the activists could face.

Two activists from Finland and Switzerland had climbed up the side of Gazprom's platform in the Barents Sea in the Russian Arctic on August 18 to protest its oil drilling.

The two were detained after warning shots were fired. They and the entire crew were later placed under arrest and locked up in the Arctic Sunrise's mess.