ANCHORAGE, Alaska (AP) — A judge on Sept. 7 temporarily halted lease sales of more than 1 million acres in the National Petroleum Reserve-Alaska that environmentalists say are essential feeding and breeding grounds for caribou and migratory birds.

Nearly 13 million acres of the reserve in northern Alaska are available for lease sale or have been sold to oil companies, most notably ConocoPhillips. The company hopes to augment waning crude stocks in the Prudhoe Bay fields east of the NPR-A.

Environmentalists filed the lawsuit against the Department of the Interior, the state of Alaska and oil companies in hopes of cordoning off about 600,000 acres of the 23-million-acre reserve from more exploratory drilling. The government had planned to open bids on Sept. 27 for about 1.7 million acres, which encompass the area targeted by environmentalists.

ConocoPhillips has its eye on the contested area, which holds a potential 2 billion barrels of oil beneath the permafrost near Lake Teshekpuk.

The order, filed in U.S. District Court in Anchorage, said the government had not adequately considered the cumulative environmental effects of the lease sales in the eastern and western sections of the reserve.

Environmental impact statements addressed the effects of leasing individual parcels, but those reports were too narrow in scope because they did not consider how leasing in the northeastern part of the reserve would affect land and wildlife in the northwestern section, according to the order.

U.S. District Judge James Singleton chastised the defendants for the oversight, writing that they “violated the National Environmental Protection Act.”

The Department of the Interior and ConocoPhillips did not immediately return messages.

Singleton is expected to make a final ruling the last week of September, said Charles Clusen, director of the Alaska project for the Natural Resources Defense Council based in Washington, D.C., one of the plaintiffs.