A federal judge has made it more difficult for representatives of Greenpeace USA to board Shell Oil's drilling vessels.

In March, Judge Sharon Gleason in Anchorage ordered the group to stay a kilometer away from Shell Oil's drilling ships destined for Arctic waters off Alaska's northern coast. The restrictions applied to U.S. territorial waters up to 12 miles from shore.

On Tuesday, Gleason extended the restrictions to 200 miles offshore. Shell intends to drill 18 miles off the Beaufort Sea coast this summer, and 70 miles off the Chukchi coast.

Shell sought the preliminary injunction after Greenpeace New Zealand activists, including actress Lucy Lawless, in February boarded the Shell drill ship Noble Discoverer before it left for the U.S. West Coast for cold-weather modifications.