The vast majority of oil from the BP spill in the Gulf of Mexico is gone, President Barack Obama's energy adviser said Wednesday, and the remaining oil reportedly does not pose a serious threat.

The U.S. government will release a report that says scientists had determined that containment, burning, and skimming measures worked in dealing with the oil spill, said Carol Browner, energy and climate change adviser to President Barack Obama.

"The good news is that the vast majority of the oil appears to be gone," she said on ABC's "Good Morning America" show. "That's what the initial assessment of our scientists is telling us."

The scientists said about 25 percent of the oil had not been captured or evaporated and there still would be some tarballs washing up onshore but the government would make sure those were cleaned up as quickly as possible, Browner said.

"We do feel like this is an important turning point," she said.

Browner's comments came as BP said it had reached "a significant milestone" in its effort to permanently plug the well, which has spewed millions of barrels of crude into the Gulf of Mexico. Crews pumped heavy drilling mud into the well and now will try to seal it with cement.

The New York Times said the government report was expected to say that what is left of the oil is so diluted that it does not seem to pose much additional risk.

Most is light sheen at the surface or dispersed below the surface and federal scientists believe it is breaking down rapidly, the newspaper reported.

The report on the spill, by federal scientists with outside help, is the result of an effort to determine the total volume of oil released and to figure out where it went, the Times said. The National Oceanic and Atmospheric Administration was the lead agency on the report, the newspaper said.