Twenty Greenpeace activists chained themselves to an icebreaker in Helsinki's harbour on Tuesday in a bid to block Anglo-Dutch oil giant Shell's plans to drill for oil in the Arctic.

The activists want "to try to prevent the ship from leaving for Alaska by peaceful means," Greenpeace Nordic spokesman Juha Aromaa told AFP.

The Alaska-bound Nordica icebreaker, a Finnish state-owned ship which has been contracted to Shell, was due to leave Helsinki "today or in the coming days," Aromaa said.

The protesters, who hail from 10 countries, were "prepared to stay on the ship for a longer period" if necessary, he added.

Finnish police and the coast guard were at the scene.

"We are trying to negotiate with them to get them to leave the ship willingly," Helsinki police chief inspector Kirsi Kainth said, describing the scene as calm.

The activists had unfurled a banner on the bridge reading "Stop Shell -- Save the Arctic", Aromaa said.

"For the first time in our history we are faced with the possibility of a world without ice at the North Pole and without a home for polar bears, narwhals and walrus," Finnish Greenpeace activist Maria Hukkamaeki said in a statement.

"It is fundamentally wrong that Shell is making money off of a global catastrophe of its own creation.

"I am here onboard this ship to say no to Arctic drilling and call for the protection of one of the world's most fragile and beautiful environments," she added.

Aromaa said Greenpeace had previously been in contact with Shell to get it to halt its plans to drill exploration wells in the Arctic, but it has had no contact with the company since the start of the action in Helsinki on Wednesday morning.

The Nordica was due to join its sister ship, the Fennica, to support two drill ships en route to the north coast of Alaska to drill five exploratory wells for Shell in the Chukchi and Beaufort Seas respectively, according to Greenpeace.

The Fennica left Helsinki in March, Aromaa said.