Russian border guards were on Monday towing a ship of the environmental lobby group Greenpeace to an Arctic port where its activists could face charges for a protest on an oil rig owned by the Gazprom energy giant.

The Arctic Sunrise ship, which Russian security forces have controlled since storming the vessel in a dramatic helicopter operation on Thursday, is to arrive in the Russian Far Northern port of Murmansk on Tuesday, the group and officials said.

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, which carries a prison term of up to 15 years in Russia.

"It's looking like a Tuesday morning arrival. The ship has slowed down, due to weather conditions we believe," a Greenpeace spokesman told AFP earlier in the day.

A Russian security source quoted by the Interfax news agency also said that the ship was expected in Murmansk on Tuesday, without giving a time.

The Russian authorities said that the Arctic Sunrise was attached to a Russian tugboat to be taken into Murmansk after the captain refused to steer it himself.

Two Greenpeace activists from Finland and Switzerland had climbed up the side of Gazprom's Prirazlomnaya platform in the Barents Sea in the Russian Arctic early Wednesday to protest its oil drilling in a hugely sensitive environment.

The two activists were detained after warning shots were fired, although they were later taken back to the Dutch-flagged Arctic Sunrise where the entire crew was placed under arrest and locked up in the mess.

Agents from Russia's Federal Security Service (FSB) seized control of the ship after descending onto the deck in a commando-style operation on ropes from helicopters.

'Like Somali pirates'

Greenpeace argues that Russia had no legal right to seize the ship, saying the boarding happened outside Russian territorial waters in the Russian Exclusive Economic Zone.

Leading Russian and ex-Soviet ecological organisations on Monday published an open letter addressed to Russian President Vladimir Putin urging the release of the activists.

The letter, which was signed by the director of the Russian branch of the World Wildlife Fund (WWF) Igor Chestin, said there had long been concerns about the project and Gazprom had failed to "fulfil its promises" of a dialogue with civil society.

The letter said the arrest of the activists appeared "especially cynical" ahead of a major forum on the Arctic in the Russian Arctic town of Salekhard this week due to be attended by Putin and the presidents of Finland and Iceland.

Amid protests organised by Greenpeace outside Russian embassies across the world to sound the alarm over the plight of the activists, Kremlin chief of staff Sergei Ivanov at the weekend launched a staunch defence of the reaction of the Russian authorities.

He said the Greenpeace activists had behaved "too radically" for such a well-known organisation and compared their actions to the pirates who have wreaked havoc off the coast of Somalia.

"Many in Russia believe that this is piracy and piracy in the Somali style. And they used boat hooks no worse than the Somali pirates," he said, quoted by Russian news agencies at a conference in Stockholm.

Gazprom intends to start production from the Prirazlomnaya platform in 2014 and Greenpeace says the condition of the rig raises the risk of an oil spill in an area with three nature reserves that is home to polar bears, walruses and rare seabirds.

The world's largest gas firm, Gazprom has expanded its oil production operations in recent years and describes the Prirazlomnoye oil field as an essential element of its oil business development strategy.