ANCHORAGE, Alaska (AP) - A major fuel spill from a grounded freighter that split in two threatens sensitive wildlife habitat in the Aleutian Islands, but finding six crew members lost at sea remains a priority despite diminishing odds for their survival.

Thousands of gallons of heavy bunker fuel and diesel spilled from a soybean freighter ripped in half off the shore of Unalaska Island on Wednesday.

The area is near a wildlife refuge that is home to sea lions, harbor seals, sea otters, tanner crabs, halibut and kelp beds, according to Alena Syverson of the Unalaska/Port of Dutch Harbor Convention and Visitors Bureau.

With resources scarce in the remote and harsh area 800 miles southwest of Anchorage, the search took precedence over the environmental threat.

"There are only so many boats and so many planes, and they have been directed to the search for life," Kurt Fredriksson, acting commissioner for the state Department of Environmental Conservation, said Thursday.

U.S. Coast Guard rescue crews searched into the night, with officials saying they had not given up on finding someone alive. Estimates put survival in the 43-degree waters of the Bering Sea at about three hours.

No sign of the missing crew had been found as of early today, a Coast Guard spokesman said. Chief Petty Officer Roger Wetherell in Anchorage said there were no aircraft searching in the dark, but that at least one cutter was at the scene of the grounding.

The six crew members were plunged into the sea when a Coast Guard helicopter crashed at about 6:15 p.m. Wednesday while evacuating them from the freighter.

Four others, including three Coast Guard personnel, were rescued from the water that evening and were in good condition Thursday.

The Coast Guard said the cause of the crash was still unknown, and that a board was being created to investigate the accident.

The 738-foot Selendang Ayu was cleaved neatly in two, both pieces grounding about 200 feet from the shore near Skan Bay on the western side of the island. Farther upshore lay the wreckage of the Coast Guard helicopter.

The freighter belongs to Singapore-based IMC Group.

Rescuers battled rolling seas, 30-knot winds and the thin Alaska daylight. On Thursday evening, officials said boats would continue the search into the night.

Fredriksson said it was not known how much fuel had leaked, but called it a major spill that could take months to clean up, threatening fragile wildlife habitats.

"You've got bunker oil streaming from a ship that's broken in half," he said. "We are in winter and in a very difficult Aleutian Island environment that will put everybody to the test."

IMC has contracted a private spill response company, Fredriksson said.

He said the rough seas could help break up some of the oil and disperse it to the open sea.