Media playback is unsupported on your device Media caption Dr Alexander Travis of Cornell University, New York (Video courtesy of Cornell University)

The world's first "test tube" puppies have been born after years of attempts, say scientists in the US.

The in-vitro fertilisation success paves the way for conserving endangered breeds and could help in the fight against human and animal diseases, say researchers at Cornell University.

The seven beagle and cross-bred beagle-spaniel puppies were born to a surrogate mother.

They were from the same litter but have three sets of parents.

Frozen embryos were implanted in a female dog using techniques similar to those used in human fertility clinics.

Problems with freezing embryos have caused difficulties in the past, but the group say they have perfected this and other techniques.

Rare breeds

Lead researcher Dr Alex Travis, from Cornell's college of veterinary medicine, said: "We have seven normal happy healthy puppies."

He added: "Since the mid-1970s, people have been trying to do this in a dog and have been unsuccessful.

"Now we can use this technique to conserve the genetics of endangered species."

Image copyright Jeffrey MacMillan/Cornell Image caption Cornell scientist Jennifer Nagashima with a beagle-spaniel pup

The researchers say IVF is a powerful tool to help endangered species of dog such as the African wild dog.

It could also be used in the study of inherited human and dog diseases.

Dogs share many similar diseases with humans - almost twice as many as for any other species.

Dr Travis said the work was an important milestone.

"In vitro fertilisation is a really powerful tool to help preserve endangered species of dog," he told the BBC.

"IVF is also important for the health of our pets because it opens up the possibility that we could identify certain genes that cause disease and then fix those."

Secret birth

The puppies were born in the summer.

Their existence was kept secret until the findings were formally announced to the scientific world this week.

They have reportedly been named Ivy, Cannon, Beaker, Buddy, Nelly, Red and Green, and all but one has gone to a new home.

Image copyright Jeffrey MacMillan/Cornell Univ Image caption The puppies were born in the summer

The research, published in the journal PLoS One, has been described as a "major step forward" in medicine.

Prof David Argyle, head of the school of veterinary medicine at the University of Edinburgh, which was not part of the study, said the new techniques would help understanding of inherited diseases in both dogs and people.

"Importantly, it is becoming apparent that dogs and humans share many common biology, diseases and syndromes, and it is likely that these new techniques could have significant benefit for the study of human diseases as well as canine diseases," he added.

Follow Helen on Twitter.