Thousands of migrating geese have died after taking refuge from a snowstorm in toxic mine waters in the western United States, mine officials have said.

As many as 10,000 snow geese descended on the abandoned open Berkeley Pit in Butte, Montana on Nov 28 and since then several thousand have perished, said Mark Thompson, environmental affairs manager for Montana Resources, which jointly manages the pit with the Atlantic Richfield Company.

Thompson said that mine workers tried to prevent the huge flock from landing on the water - which contains sulfuric acid and heavy metals - by using noise makers, spotlights and other methods but failed to scare them all off.

He described the pit's 280-hectare lake as "white with birds" when the flock landed on the water.

Since then, area residents have found dead birds in parking lots, in front of a casino, on the roadside and outside town. Thousands more are estimated to have died in the lake, officials said.

"Trying to get some idea of mortality has been difficult," Joe Vranka of the Environmental Protection Agency, told the Billings Gazette.

Meanwhile, the company and the EPA are keeping a watch to prevent any more flocks headed toward Butte from landing in the water.

This is not the first time geese have died at the pit, which ceased operating in 1982 and has since been submerged with toxic water.

In 1995, 342 dead geese were found floating in the pit.

EPA officials said the companies that manage the mine will be fined if found not in compliance with a "bird hazing" program designed to prevent animals from spending too much time in the water.