The first person to receive a genetically modified pig's heart has died two months after the groundbreaking operation, the University of Maryland Medical Center said Wednesday.

David Bennett, 57, received a pig heart on Jan 7 at the hospital and had been recovering, but his condition began deteriorating several days ago, the hospital said in a statement.

It isn't yet exactly clear what caused Bennett's death, according to the hospital, which said the transplanted heart performed "very well for several weeks without any signs of rejection".

Bennett was given "compassionate palliative care" when it became clear that he wouldn't recover. He was able to communicate with his family during his final hours, the hospital said.

After the surgery, Bennett was able to spend time with his family and participate in physical therapy to help regain strength. He watched the Super Bowl with his physical therapist and he often told his caregivers how much he wanted to get home to see his dog, Lucky.

"We are devastated by the loss of Mr. Bennett. He proved to be a brave and noble patient all the way to the end,'' Dr Bartley Griffith, who performed the operation, said in a statement.

Bennett was deemed ineligible for a conventional heart transplant though he had severe heart disease. On New Year's Eve, the Food and Drug Administration granted an emergency authorization for the experimental surgery, which was performed a week later.

The pig heart he received had been modified to remove pig genes that trigger hyperfast rejection and human genes added to help the body accept the organ. Prior attempts at such transplants — or xenotransplantation — have failed largely because patients' bodies rapidly rejected the animal organ.

Bennett's transplant is still considered a significant step forward, because the pig heart wasn't immediately rejected and continued to function for well over a month, passing a critical milestone for transplant patients.

Bennett survived significantly longer with the gene-edited pig heart than one of the last milestones in xenotransplantation, when Baby Fae, a California infant, lived 21 days with a baboon heart in 1984. Her immune system rejected the heart.

"We have gained invaluable insights learning that the genetically modified pig heart can function well within the human body while the immune system is adequately suppressed," Muhammad Mohiuddin, an expert in interspecies transplants who was involved in Bennett's care and at the University of Maryland School of Medicine, said Wednesday. "We remain optimistic and plan on continuing our work in future clinical trials."

Bennett's heart transplant surgery was one of several pioneering procedures in recent months to use organs from genetically modified pigs to replace human organs. Doctors are looking at the process in hopes of helping solve the shortage of donated human organs.

Some 41,354 Americans received a transplanted organ last year, including about 3,800 heart transplants, and more than half were kidneys, according to the United Network for Organ Sharing, a nonprofit that coordinates the nation's organ procurement efforts. More than 100,000 people are on the national waiting list for a transplant.

Bennett's son, David Bennett Jr, said he hopes his father's experience will eventually help many others live far longer with transplanted pig organs.

"We hope this story can be the beginning of hope and not the end," he said. "We also hope that what was learned from his surgery will benefit future patients and hopefully one day end the organ shortage that costs so many lives each year."