Thirteen environmental activists rappelled off a bridge Wednesday in Portland, dangling from ropes to get in the way of a Shell Oil ship headed for Alaska on a drilling expedition, Greenpeace said.

The activists are hanging from the St Johns Bridge in Oregon's main city to prevent the departure of the MSV Fennica, a Shell icebreaker that was in town for repairs.

The activists have enough supplies for several days, Greenpeace said.

And they are "prepared to stay in Shell's way as long as possible," the environmental group said in a statement.

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.

One of the danglers, Dan Cannon, posted a photo on Twitter from above the river, showing his feet, red hammock, the water and the pale green underbelly of the bridge.

"Setting up camp is hard work but the view from the hammock is worth it," Cannon wrote.

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.

The ship was supposed to have left just before dawn but it has now put it off for five hours, until 1700 GMT, local TV station Katu reported.

Traffic continues to flow across the bridge but police were denying access to pedestrians.

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.