US scientists have cloned a rare breed of pig in an experiment they say shows the copying technology can be made more efficient.

The cloned animal, called Princess, was the last female in one of only four bloodlines of Gloucestershire Old Spots in North America.

Several attempts at getting offspring from Princess through natural breeding and artificial insemination had failed, NewScientist.com reports.

Cloning produced three piglets, born via a surrogate mother, after just one embryo implantation round.

Genetic diversity

Two of the clones, now a couple of weeks old, are alive and appear healthy; one was accidentally killed by the surrogate.

Scientists see a role for pig clones as organ suppliers for human transplants

Although some scientists are enthusiastic about the potential of animal cloning in drug and organ production, for example, the technology's use for conservation work has fewer supporters.

But for Robyn Metcalfe, founder of the Kelmscott Rare Breeds Foundation in Maine, cloning was the obvious way to salvage a bloodline that might have otherwise gone extinct in the US.

"We're fully aware that cloning won't increase genetic diversity, but in this one case, if we can reproduce Princess by cloning, we can bring that diversity back," he told NewScientist.com.

Don Bixby, director of the American Livestock Breeds Conservancy, said cloning "might help for these dead ends, but it's not something we would normally choose".

Difficult procedure

He said there was some evidence suggesting that a percentage of cloned animals might suffer from genetic defects.

The company behind the Princess cloning said the project showed how much more efficient it had made pig copying.

After fusing skin cells from Princess with donor eggs to create the clones, only one implantation procedure was required. The surrogate sow was implanted with between 100 and 150 embryos and a successful triplet pregnancy resulted.

When scientists first began their attempts to clone pigs, they found it extremely difficult to get the surrogate mothers - which give birth to litters - to carry the implanted embryos.

The world's first pig clones were born on 5 March, 2000 - four years after Dolly the sheep clone's birth.