Shell said there are a series of contingencies to consider after a U.S. federal decision to reaffirm a lease for the arctic waters off the coast of Alaska.

The Department of Interior this week affirmed a 2008 lease sale for exploration in the Chukchi Sea off the coast of Alaska. The decision clears the way for a formal review of exploration plans in the region, which will include an environmental analysis.

The lease was tied in up the court system amid complaints about the extent of environmental vetting.

Shell, which devoted about $5 billion and more than eight years of work for its arctic oil exploration off Alaska's coast in the Chukchi and Beaufort seas, said the decision clears the way for a review of its plans for offshore Alaska.

"The execution of that plan remains contingent on achieving the necessary permits, legal certainty and our own determination that we are prepared to explore safely and responsibly," a spokesperson told UPI in response to email questions.

Shell Chief Executive Ben van Beurden last month acknowledged there were "technical, fiscal, regulatory, political" and other issues that may interfere with frontier development in the arctic. He added, however, he expected the Interior Department would finish its review in time to resume drilling in 2015.

The Consumer Energy Alliance, which supports the interests of the fossil fuels industry, said in an emailed statement the federal government needs to vet the lease in a way that doesn't get in the way of Alaska's energy development. By the group's estimate, waters off the coast of Alaska hold an estimated 27 billion barrels of oil and 132 trillion cubic feet of natural gas.

Environmental groups this week cried foul after the Interior Department announced its decision. Advocacy group Friends of the Earth said it was "unconscionable" for the government to consider opening a sensitive environment up for the "reckless pursuit of oil."