WASHINGTON — Many scientists say they're skeptical of a widely publicized government report Wednesday that concludes much of the oil that gushed from BP's leaking well is gone and poses little threat to the Gulf of Mexico.

According to the National Oceanic and Atmospheric Administration, the "vast majority" of the 4.9 million barrels released into the Gulf has either evaporated "or been burned, skimmed and recovered from the wellhead, or dispersed."

"I'm suspect if that's accurate or not," said Ronald Kendall, the director of the Institute of Environmental and Human Health at Texas Tech University and one of the scientists who testified Wednesday at a congressional hearing about the need for more research into the composition and use of chemical dispersants to break up the oil in the Gulf.

"It's an estimate, and I'd like to say that even if it's true, there are still 50 to 60 million gallons that are still out there," he said. "It's too early to draw the conclusion that the coast is clear, but there are species there that will tell us."

The White House used the report to boost public confidence that the accident at BP's drilling site, which killed 11 workers, fouled the Gulf, killed wildlife and disrupted the regional economies from Texas to Florida, is now behind the nation.

Many scientists, however, questioned the rosy White House assessment and the administration's motives, timing and record of estimating how much oil was flowing from the well.

The report says that less than half of the oil remains in the environment. About 26 percent of it remains as surface sheen or tar balls, or other forms of oil. About 16 percent was dispersed naturally, and another 8 percent was dispersed by the chemicals BP pumped onto the surface of the Gulf and deep underwater at the source of the leak.

Jane Lubchenco, NOAA administrator, said at a White House press conference that "much of the dispersed oil is in the process of relatively rapid degradation."

Obama press secretary Robert Gibbs; the president's energy and climate change adviser, Carol Browner; and Thad Allen, who as the national incident commander directed the BP cleanup, joined Lubchenco at the White House briefing.

President Obama, addressing the AFL-CIO on jobs, expressed optimism that "the long battle to stop the leak and contain the oil is finally close to coming to an end." Early Wednesday, BP reported that it had stopped the leak by pumping heavy drilling mud into the well shaft.

The scientists and other experts who challenged the government's conclusions warned that painting too rosy a picture could hamper the environmental monitoring and cleanup work that remains to be done in the Gulf.

Marine conservationist Rick Steiner, a retired University of Alaska scientist, said: "Let's look at this another way: that there's some 50 percent of the oil left. It's still there in the environment."

The government report also fails to account for the effect of vast, underwater plumes of microscopic droplets of oil that remain unmeasured, scientists said, and it downplays the potential long-term effects of the release of as much as 4.1 million barrels of oil in the Gulf of Mexico. Some 800,000 barrels were captured at the wellhead.

The remaining 50 percent in the water is the equivalent of almost eight Exxon Valdez oil spills, until now the country's benchmark environmental disaster.