Greenpeace protesters dangling from a bridge in Portland, Oregon, on 31 July failed to prevent an icebreaker from passing through en route to the Arctic.

Greenpeace protesters, who rappelled down from the bridge over the Willamette River had hoped to shorten Shell's Arctic drilling season by stopping the Fennica icebreaker, which is carrying emergency equipment that would cap any blown-out well.

Shell is not allowed to start drilling without it and the drilling season ends in October, when sea ice forms.

Greenpeace says Arctic drilling could be damaging to populations of whales, polar bears and walrus in the event of an oil spill.

The Fennica stalled once earlier on Thursday as it neared the St John's bridge on the Willamette, which leads to the Pacific Ocean. Protesters dangling from ropes with large red and yellow banners, at times chanting "Shell No", prevented the ship from passing under the span, forcing the vessel back to port.

Dozens of kayakers were pushed to one side of the river by waterborne US Coast Guard backed by sheriff's deputies on loud speakers. The bridge and the river were closed to traffic.

Onlookers sympathetic to the protests gathered in a nearby park and occasionally cheered in support. Two activists were issued police citations, Greenpeace said.