Frustrated by furious winds, mountainous seas and a mere five-hour window of December daylight, rescuers searched Thursday for six people lost in the Bering Sea (search) after the Coast Guard helicopter that had plucked them from a crippled freighter crashed in the darkness.

The ship they left behind ran aground and split apart, and fuel from the vessel had started leaking. Environmental officials feared that it would become a massive spill.

Searchers hoped the missing crew had somehow lived through the night, but 43-degree waters reduced survival estimates to about three hours. Rescuers were hampered by seas that swelled to 20 feet and wind that howled at 35 mph.

"Those high winds make flying difficult and is going to make for choppy seas," Coast Guard (search) Chief Petty Officer Roger Wetherell said. "If somebody is out there riding a wave, you may not see them."

Searchers reported improving visibility, but limited daylight forced them to wait until after sunrise at 10 a.m. to resume rescue flights.

The rescue helicopter crashed into the sea Wednesday carrying 10 people — seven crew members and three Coast Guard personnel. Four were rescued by a second helicopter and in good condition Thursday. The cause of the crash was not immediately known.

The missing crew members included one Filipino and five people from India.

The freighter's captain and a rescue swimmer who had remained on the vessel were also rescued shortly before the ship broke up. Eighteen other crew members were previously evacuated.

Rear Adm. James Olson said he did not know whether the missing crew members were wearing survival gear.

"The six crew members are still unaccounted for and there is fuel in the water," Wetherell said.

The 738-foot freighter was carrying 480,000 gallons of heavy fuel oil and 21,000 gallons of diesel fuel on board along with its cargo of soybeans that were destined for China.

The ship disintegrated off the coast of Unalaska Island (search), about 800 miles southwest of Anchorage in a region that is home to sea lions, northern fur seals, and a variety of birds and other sealife.

Officials were still trying to determine how much of the fuel spilled.

Lynda Giguere, a spokeswoman for the state Department of Environmental Conservation, said it is a dense, viscous oil that is not easy to clean up. "It's not good stuff," she said.

The Coast Guard had been struggling to help the 40,000-ton freighter since Tuesday when it began drifting after its main engine failed, but bad weather hindered the efforts.

A tug boat had attached a line to the freighter on Tuesday evening, securing it for 12 hours. But then the line broke and the vessel resumed its path to the Unalaska Island shore.

The crew dropped anchor when it reached shallow water, but it was lost in the rough seas after just a half hour.

The crew later dropped its other anchor, which for a while held the freighter nearly a mile from shore, Olson said.

But sometime around 6 p.m. Wednesday, the captain of the freighter requested the remaining crew members be evacuated from the vessel, as the anchor had begun to give way and the freighter had started to flood.

The helicopter crashed into the sea soon after picking up crew members. Around 7:15 p.m., the freighter broke in half.

Retired fisherman Ben Golodoff heard radio reports about the disaster Wednesday and recalled his days battling the region's unruly seas on frigid, pitch-black nights.

"Out there, you've got the cold weather of the Bering Sea to the north and to the south you've got the Pacific — and where they both meet around this area, it can get pretty turbulent."

The freighter was registered under a Malaysian flag and owned by Singapore-based IMC Group.

The company's crew manager in Singapore, Loh C.W. Weng, said agents in India and the Philippines had contacted families of the crew.

"Of course, they feel very sad and want to know what is going on. They are praying very hard that everyone is OK. We are praying very hard for them," Weng said.