It may never to possible to clone humans because of a quirk in our biology, scientists say.

Researchers in the United States were trying to get to the bottom of why hundreds of attempts to clone monkeys failed. They found from the start, cloned primate cells failed to divide properly.

Dozens of animal clones, including cows, pigs, mice, goats and a cat, have been born since Dolly the sheep became the first mammal to be born from an adult cell in 1997.

Many cloned animals are born stillborn and some survive with severe defects. The American team tried the Dolly cloning method on 724 eggs from female rhesus macaques. The experiment yielded only 33 embryos and not a single pregnancy.

Lead researcher Gerald Schatten of the University of Pittsburgh suspected chromosomal abnormalities could be to blame. When the team looked inside cloned monkey embryos, they found deformities.

A key structure called the mitotic spindle is needed to properly line up and separate chromosomes before they are pulled apart into two cells. If chromosomes don't line up properly in humans, defects such as Down syndrome can result.

Eggs have proteins that act as molecular motors for the spindles. The proteins are tightly bound to the egg's DNA. The first step of cloning appears to pull the proteins apart, dooming the later pregnancy, Schatten said.

"Current techniques such as those used to create Dolly the sheep, mice and other domestic animals do not work in non-human primates," Schatten said.

The study casts further doubt on claims of cloning human babies.