Swedish police said they arrested six Greenpeace activists who boarded an icebreaker in the Baltic Sea Thursday in a bid to block plans by Anglo Dutch giant Shell to drill for oil in the Arctic.

The activists, who boarded the Finnish icebreaker Nordica, were arrested for "aggravated trespassing and criminal conversion," police said in a statement.

The Nordica is under contract to Shell with another icebreaker, the Fennica, to provide support for an operation to drill five exploratory wells in the Chukchi and Beaufort Seas respectively, Greenpeace says.

Shortly after 4:00 am (0200 GMT) Thursday, "six Greenpeace activists from Finland, Israel, Austria, Sweden and Denmark boarded the Nordica south of the Swedish island of Oeland," Greenpeace Nordic spokesman Juha Aromaa told AFP earlier.

A Greenpeace video showed the activists in colourful padded jumpsuits and helmets speeding up alongside the icebreaker in small rubber boats before scaling the side of the ship and hanging up banners with "Stop Shell" and "You can save the Arctic".

According to local officials, Swedish authorities boarded the icebreaker at around 8:30 am. They had arrested one of the activists, but the five others had remained chained to the vessel until the ship was piloted into port in the southeastern town of Karlskrona.

Finnish police said 40 Greenpeace demonstrators were detained Tuesday after they tried to block it from leaving Helsinki.

Its sister ship, the Fennica, left Helsinki in March.

Environmentalists have pointed to the vastly complicated task of drilling in the harsh Arctic environment, the difficulty of effectively cleaning up any spills in such conditions, and the risks posed to wildlife and native communities in the region's fragile ecosystem.