The Interior Department finalized its environmental review for ConocoPhillips' Willow project Wednesday, laying the groundwork for approval next month of what would be the nation's largest oil development on public lands.

Interior's supplemental environmental impact statement, released Wednesday, included a preferred alternative that would give developers up to three drill sites in the National Petroleum Reserve in Alaska.

ConocoPhillips had sought to set up five drill sites.

Interior stressed that its supplemental environmental impact statement is not a final decision, saying that it has "substantial concerns about the Willow project" and the preferred alternative relating to its prospective direct and indirect greenhouse gas emissions and effects on wildlife and Alaska Native subsistence.

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