The remote refuge stretches over 19 million acres (7.7 million hectares) and holds up to 16 billion barrels of oil. President George W. Bush, a former Texas oilman, and many of his fellow Republicans back drilling there to boost U.S. energy supplies.

Environmental groups oppose the plan, saying drilling would destroy a scenic place sometimes called "America's Serengeti" and would also fail to yield any sizable amount of oil for several years.

According to the government report, drilling in the refuge could especially hurt the Porcupine River caribou herd, which travels some 400 miles from Canada's Yukon Territory to the Alaskan coastal plain for calving in May and June.

The herd, which has dwindled to an estimated 123,000 animals, uses the entire coastal plain area which the Bush administration wants to open to drilling.

Pregnant caribou avoid roads and pipelines and calves have "repeatedly shown to be sensitive to disturbance," it said.

"Oil development will most likely result in restricting the location of concentrated calving areas, calving sites and annual calving grounds," the report said. "Expected effects that could be observed include reduced survival of calves during June, reduced weight and condition of (pregnant) females and reduced weight of calves in late June."

An Interior Department spokesman downplayed the report, saying it was based on an outdated drilling plan that included a major highway, an airport and "intensive" energy production.

The report's conclusions "are not based on the reality of the current legislation proposed in congress," said Interior spokesman Mark Pfeifle. "Neither the highway or the airport will ever be constructed because ice roads and ice runways will be used. They disappear in the springtime when the caribou are calving."

GEESE, OXEN ALSO AT RISK

Snow geese and musk oxen are among other wildlife that could also be affected, the report said.

The geese have a small area for feeding on the coastal plain, making them vulnerable to oil drilling activities.

Oil exploration may also hurt the area's musk oxen, which survive temperatures as low as minus-40 degrees Fahrenheit by reducing their movements to conserve energy. The estimated 300 oxen living year-round in the plain may be weakened if they are forced to move away from a drilling area, the report said.

The Arctic refuge is also used by pregnant polar bears and as a nesting area for several kinds of migratory birds.

While Bush and many Republicans have repeatedly endorsed drilling, the U.S. Energy Information Administration said recently that it would take about two decades before any crude oil pumped from the refuge could reduce U.S. oil imports.

The Democratic-led Senate is set to debate Alaska drilling when lawmakers return from a spring vacation on April 8.

A Reuters survey of all 100 U.S. senators earlier this month found that the White House proposal to open the refuge to drilling appeared doomed in the Senate. At least 50 senators, including five Republicans, said they opposed drilling in the Arctic National Wildlife Refuge, and 10 others were undecided.

Under the Senate's rules for controversial legislation, 60 votes are required to cut off debate and proceed with a vote.

Sen. John Kerry, a Massachusetts Democrat, said the new report showed why drilling is a bad idea.

"The administration should take to heart the conclusive scientific findings of the U.S. Geological Survey and finally put aside plans to drill in the pristine wilderness of the Arctic," Kerry said in a statement.