"We know little about the viruses carried by pigs, so transplanting a pig organ into a human could add to the risk of animal epidemic diseases infecting humans," said Lu Shichun, a liver transplant expert at Beijing YouAn Hospital.

Organs from GM pigs have been used in tests in the United States since 2002 where they have sustained the lives of monkeys for up to 400 days. Such temporary transplants could potentially help keep human patients alive while waiting for human transplant organs.

Scientists from the Chinese Academy of Agricultural Sciences first bred GM pigs in November 2010. Clinical trials on humans have not been carried out anywhere in the world.

Because of a shortage of organs, only 10,000 out of 1 million people with kidney disease and 300,000 with liver disease awaiting a transplant are saved each year, Huang Jiefu, vice-minister of health, revealed earlier.

The research group will apply for permission to conduct clinical experiments after the piglets grow, said Dai.

Scientists said it is not known whether the government will allow experiments on transplanting pig organs into humans.