Russia in a "surprise move" is to release Greenpeace's Dutch-flagged Artic Sunrise ship, seized last year after a protest against Arctic oil drilling, the environmental group said on Friday.

"Russia's investigative committee this morning informed Greenpeace International that it has annulled the arrest of the Arctic Sunrise, which has remained in custody in Murmansk," Greenpeace said in a statement.

Russian authorities however told Greenpeace's lawyers that investigations continued "in order to examine equipment found on board the ship."

Russian commandos seized the Arctic Sunrise in September last year and detained 30 Greenpeace activists and journalists after a protest at an offshore oil rig owned by Russian state oil giant Gazprom.

Originally facing a charge of piracy, they were later targeted with less severe hooliganism accusations.

The 30, including four Russians, were detained for around two months before being bailed and then benefiting from a Kremlin-backed amnesty.

The Arctic Sunrise was towed to the northern Russian port of Murmansk.

An international maritime court in Germany in November told Russia to release the activists and the ship in response to a formal complaint lodged by the Netherlands.

Russia boycotted the hearing.

Greenpeace is now suing Russia before the European Court of Human Rights for what it says was the illegal detention of its activists as it breached their rights to freedom of expression.

Greenpeace "reacted positively to the news, but reaffirmed its belief that the arrest of the ship was illegal under international law," it said, calling the announcement a "surprise move".

"Our ship was arrested during an entire peaceful protest against Arctic drilling in international waters," Greenpeace International's director Kumi Naidoo said.

Greenpeace have asked Russia to continue to guard the ship in Murmansk "until our crew arrives to take custody of it."

But said Naidoo: "Today the final member of the group is free to come home."