By Helen Briggs BBC News science reporter

Skull of the cave bear, Ursus speleaus

They plan to unravel the DNA of other extinct species, including our closest ancient relatives, the Neanderthals.

But they say the idea of obtaining DNA from dinosaurs, depicted in the film Jurassic Park, remains science fiction.

It is highly unlikely that viable genetic material will ever be recovered from fossils that are hundreds of millions of years old.

But the scientists hope to be able to sequence the DNA of ancient humans, which lived at the same time as cave bears, raising the prospect of perhaps one day being able to "build" a Neanderthal from their genetic blueprint.

Jurassic Park

"In hundreds or thousands of years from now, we may have advanced our technology so we can create creatures from DNA sequence information," Dr Eddy Rubin, director of the US Department of Energy Joint Genome Institute in Walnut Creek, California, told the BBC News website.

Eddy Rubin: 'We were looking for the proverbial needle in the haystack'

"I don't think we can extract DNA from dinosaurs; I think they are too old. As for creating Jurassic Park, I think that remains science fiction."

The scientists extracted DNA from the tooth and bones of cave bears found at two sites in Austria.

The cave bear was once common in Europe but died out about 10,000 years ago, when the forests shrank at the end of the last Ice Age.

Dr Rubin's team analysed the extinct bear's DNA using powerful computing technology developed during the human genome project.

This approach has been hampered in the past by the fact that ancient DNA is contaminated with genetic material from bacteria and people who have handled the ancient remains.

Sifting out the ancient DNA from this genetic soup is like looking for a needle in a haystack.

DNA was extracted from a tooth

But because we now know the genetic sequences of many organisms - including numerous microbes, the human, and animals such as the dog - the researchers were able to isolate the sequences they were interested in.

In the case of the cave bear, they used the sequence of the dog, which exists in public databases, and the DNA of modern bears, as a "magnet".

Dr Rubin said it served as a "proof of principle" that the method works.

They are now turning their attention to the Neanderthals, the closest ancient relatives of modern humans, who lived around the same time as the bears.

"I think it will work," he said. "It is just a matter of time."

Mitochondrial DNA

Most fossils are of no use to scientists hunting ancient biomolecules. It is only in those specimens where some sub-fossil material - tissues that have not been fully mineralised - still exists that researchers can hope to retrieve some genetic information.

And in the last few years, a number of labs have developed the expertise to extract fragments of DNA from animals that died out tens of thousands of years ago.

It has been very difficult thus far to get anything other than mitochondrial DNA from ancient material Dr Dan Bradley

Most of these samples, though, have been from mitochondria, the structures in the cell that produce energy and have their own genetic material.

While this can provide valuable information about the evolutionary history of a species, it is the DNA within the nucleus, the nuclear or genomic DNA, which contains the bulk of an animal's genetic information, including the secrets of how modern animals differ from their ancestors.

Dr Dan Bradley, an expert on ancient DNA at Trinity College, Dublin, Ireland, said the research was "very encouraging".

"It has been very difficult thus far to get anything other than mitochondrial DNA from ancient material," he said.

"That is only a very small part of our DNA with limited interest."

The research is reported in the journal Science.