Two separate teams of researchers reported that they had managed to clone pigs, a tricky task that opens the door to breeding herds of genetically engineered pigs to farm for organ transplants to people.

But other scientists said they had found tough barriers to such an attempt--saying that they had shown that human cells can be infected with potentially dangerous viruses from pigs.

So far,sheep, cattle, goats, mice, and monkeys have been cloned. One idea is to breed genetically identical farm animals that can produce human products such as protiens for use in medicines.

But pigs offer another prospect. Because they are similar in size and other aspects of biology to humans, they have been seen as a potential source of organs and tissue for transplant into people.

Both research teams--one based in Edinburgh and the other an international group--said they hope they can genetically engineer pigs in the future so their organs and tissues are easier to transplant into human beings.

The main target will be a sugar called alpha-1,3,-galactose, which pigs and many other animals have on their cells but humans do not have. The sugar causes an extreme rejection of animal tissues by the human immune system.

Getting rid of this sugar may go a long way "towards the ultimate goal of providing an unlimited supply of compatible pig organs for human transplantation", the Edinbourgh team wrote.

But another study poured cold water on the idea that this can be done any time soon.

Daniel Salomon of the Scripps research Institute in La Jolla, California and colleagues found human cells can be infected with viruses that exist in all pig cells, known as porcine endogenous retroviruses or PERVs.

"Here we show that pig pancreatic islets produce PERV and can infect human cells in culture," they wrote.

The science journal Nature will publish the Edinbourgh teams findings later this month, while rival journal Science is publishing the paper from the international group.