One of the last significant hurdles to offshore oil drilling in the Arctic was cleared Friday with approval of a plan to deal with a nightmare scenario - an oil spill at the top of the world.

The Obama administration accepted Shell Gulf of Mexico Inc.'s plan for responding to an accident should it occur in the Chukchi Sea. The company hopes to begin exploratory drilling there, 70 miles off the northwest coast of Alaska, in June.

The issue of how to clean up a spill in the remote waters, 1,000 miles from the nearest U.S. Coast Guard base, has been the biggest impediment to opening the most significant new frontier in U.S. energy development. But Interior Department officials said Shell's plan could protect the fragile environment even in the event of a large blowout.

"After an exhaustive review, we have confidence that Shell's plan includes the necessary equipment and personnel pre-staging, training, logistics and communications to act quickly and mount an effective response should a spill occur," said James A. Watson, director of the Bureau of Safety and Environmental Enforcement.

The plan calls for a fleet of six oil-spill response vessels to be on hand at all times, along with a U.S. Coast Guard vessel. It also calls for a specially designed capping and containment system, still under construction and testing, that would be able to contain and store an accidental release of up to 80,000 barrels a day.

The worst-case flow volume anticipated from any blowout is 25,000 barrels a day, Watson said in a conference call with reporters.

He said experience with the Deepwater Horizon spill in the Gulf of Mexico prompted a number of changes to Shell's oil spill response plan for the Arctic. Among them: beefed-up well-drilling standards; the permanent presence of a full-time federal inspector on board; standards and inspections on blowout preventers; and the existence of a second drilling rig nearby to drill a relief well in the event of a blowout.

Shell, which also is applying to drill exploratory wells nearer shore in the Beaufort Sea, plans to use its Beaufort drilling rig as a backup for drilling a relief well if needed in the Chukchi, and vice versa.

Conservationists say the plan, as approved, provides no real guarantee that an oil spill would not devastate one of the world's most fragile environments, home to polar bears, walruses, whales and thousands of migrating birds.

"This is a premature decision," Marilyn Heiman, director of the U.S. Arctic program for the Pew Environment Group, said in an email to the Los Angeles Times.

She said the plan should, but doesn't, include an adequate plan for protecting the shoreline and feeding areas for migrating bowhead whales and other species in the event of a spill.

"Ideally, they need two to three more years to really do this safely - to get the science right, to protect wildlife areas and to get equipment that's been designed for and tested in the Arctic," Heiman said.

Most of the nation's biggest environmental organizations have urged a moratorium on offshore drilling in the Arctic, at least until additional scientific studies and protections are in place.

More than a dozen conservation and indigenous groups purchased a full-page ad in the Seattle Times in advance of President Barack Obama's visit to Seattle on Friday. It shows the drilling rig at the scene of the Deepwater Horizon spill in flames, and warns that an Arctic spill could be worse.

"Severely cold weather, shifting sea ice, and months of winter darkness could turn a spill here into a logistical nightmare - and an environmental disaster," the ad warns.

But officials in Alaska have long urged an expansion into one of the world's last great reserves of crude oil. They say they're convinced that offshore drilling is crucial to reducing the nation's reliance on foreign oil, boosting jobs and keeping the trans-Alaska pipeline running.

"After three years of hard work with federal agencies, we have cleared another hurdle toward oil and gas development in Alaska this summer," U.S. Sen. Mark Begich, D-Alaska, said in a statement.

Shell officials have already begun submitting drill permit applications - and hope to win approval of their oil spill response plan in the Beaufort Sea, along with final air emissions permits from the Environmental Protection Agency - in time to begin a drilling operation that has already been delayed several years.

"Approval of our Chukchi Sea oil spill response plan is another major milestone on the path to drilling in Alaska offshore this summer and further validates the huge amount of time, technology and resources we have dedicated to assembling an Arctic oil spill response fleet, which is second to none in the world," Pete Slaiby, Shell's Alaska exploration manager, said in a statement.

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