A US judge Thursday slapped a $2,500 fine on Greenpeace for every hour its activists continue to block a Shell Oil ship headed for Alaska on a drilling expedition, a court official said.

The activists have been hanging from ropes since Wednesday from a bridge in Portland, Oregon's main city, to prevent the departure of the MSV Fennica, a Shell icebreaker that was in town for repairs.

But federal judge Sharon Gleason in Anchorage, Alaska's biggest city, ruled in the oil giant's favor on Thursday, imposing the hourly fine until the protestors withdraw, a court official told AFP.

The fine will increase the longer the protest continues, according to the local KGW News.

Starting at 10 am (1700 GMT) Friday, Greenpeace will be fined $5,000 an hour, increasing to $7,500 an hour on Saturday and $10,000 an hour from Sunday.

Greenpeace's press office did not immediately respond to a request for comment.

On the green group's Twitter page it urged its supporters to sign an online petition to the White House calling on the Obama administration to rescind Shell's Arctic drilling lease.

On Wednesday Greenpeace said the activists have enough supplies for several days, adding that they were "prepared to stay in Shell's way as long as possible."

TV footage showed the activists dangling from the bridge in hammock-like devices above the water. A similar number of colleagues in kayaks are in the water below, also trying to keep the Shell vessel from leaving.

The 116-meter (380-foot) Fennica was due to leave Wednesday to join the rest of a fleet of Shell ships in the Aleutian islands, which spread west from Alaska.

It was in Portland for repairs after suffering an accident in the Arctic that left a hole in its hull. Shell cannot start drilling without the icebreaker because it is carrying a crucial piece of equipment, Greenpeace said.

In mid-June, Greenpeace activists in kayaks tried to block the departure of a giant Shell oil platform from Seattle and keep it from drilling in the Arctic.

The protests follow authorization given to Shell by President Barack Obama in May to drill for oil in the Arctic, a decision which infuriated environmental groups.