These pig clones are miniature ones (Image from Science)
It follows an announcement by Scottish-based PPL Therapeutics that genetically engineered clone piglets had been born on Christmas Day.
The second team of researchers, based at the University of Missouri-Colombia and Immerge BioTherapeutics Inc, claim they actually made the advance first.
The existence of the second litter was revealed in the journal Science. The Missouri-Columbia pigs were born about three months before those of their Scottish rivals.
A key gene that causes the human body to reject pig tissue has been switched off.
Although the same gene has been "knocked out" in both sets of pigs, the animals used are not of the same breed.
The Immerge BioTherapeutics pigs are miniature ones.
Company President Dr Julia Greenstein said: "We have been actively developing a line of miniature swine that offers many advantages as a potential donor for xenotransplantation, including their organ size, which is appropriate for human recipients."
Ethical concerns
Pharmaceutical companies are racing to develop technology that might one day allow animal organs and tissues to be transplanted into humans.
Experts believe this controversial area, known as xenotransplantation, could address the shortage of human organs for transplant. But there are many ethical and safety concerns.
They include the theoretical risk that pig viruses might be passed to humans and cause new diseases.
An investigation by British scientists into the risks of animal transplants into humans has warned that a virus called porcine endogenous retrovirus can infect human cells in the laboratory.
Immerge BioTherapeutics says its pigs have been selectively bred to minimise the risk of such viruses being passed to humans.
Dr Greenstein said: "Preliminary research shows that cells from this line of pigs, in contrast to most other cells tested, don't have the capacity to spread porcine endogenous retrovirus to human cells in culture."
'Critical milestone'
The publication of the miniature pig clones study in the journal Science follows Wednesday's revelation by PPL Therapeutics.
The commercial offshoot of the Roslin Institute announced the birth of five female piglets in a press statement. The pigs, which were born at the company's research division in Virginia, US, have been named Noel, Angel, Star, Joy and Mary.
PPL says it intends to use the pigs as part of its programme to seek a cure for human suffering from diabetes.
Dr David Ayares, Vice-President of Research at PPL's US division, said the birth of the pigs was a "critical milestone" in the company's xenograft programme.
The National Kidney Research Fund in the UK welcomed the news. But it said the prospect of an abundant supply of animal organs for humans with failure of the kidneys, heart, lungs, liver, or pancreas was still "a long way off".
The fund said in a press statement: "For the foreseeable future, there remains an urgent need for human donors."
It urged everyone to join the NHS Organ Donor Register and to tell their family about their wishes.