Environmental activists who protested at an offshore oil platform in the Russian Arctic last week will be prosecuted, possibly for piracy, which is punishable by up to 15 years in jail, according to Russian investigators.

They said the "attack", in which Greenpeace activists tried scaling the Gazprom-owned Prirazlomnaya platform, Russia's first offshore Arctic oil platform, had violated Russian sovereignty.

The protest on September 19 ended when armed officers boarded the Netherlands-registered icebreaker and arrested the 30 activists onboard, Greenpeace said. The vessel, the Arctic Sunrise, was towed to the port of Murmansk.

Greenpeace says its protest, aimed to draw attention to the threat oil drilling poses to the fragile Arctic ecosystem, was peaceful, and that Russia's actions violated international law.

A diplomatic delegation of 18 officials representing nine countries went on board the Arctic Sunrise on Tuesday afternoon to meet the detainees, Greenpeace said.

The Netherlands, where Greenpeace International is based, called for the release of the ship and all activists, including two Dutch citizens.

Greenpeace said the activists had been denied access to lawyers and that they had not yet been formally charged.

"Any charge of piracy against peaceful activists has no merit ... we will not be intimidated or silenced, Greenpeace's global chief Kumi Naidoo said, adding that "peaceful activism is crucial when governments around the world have failed to respond to dire scientific warnings."

Greenpeace says scientific evidence shows that an oil spill from Prirazlomnaya would affect more than 4,800km of Russia's coastline.

Global majors including ExxonMobil, Eni and Statoil have agreed deals with Russia's state-owned Rosneft to enter Russia's Arctic offshore waters.

Source: Al Jazeera and agencies