A US man has become the first person in the world to get a heart transplant from a genetically-modified pig.

David Bennett, 57, is doing well three days after the experimental seven-hour procedure in Baltimore, doctors say.

The transplant was considered the last hope of saving Mr Bennett's life, though it is not yet clear what his long-term chances of survival are.

"It was either die or do this transplant," Mr Bennett explained a day before the surgery.

"I know it's a shot in the dark, but it's my last choice," he said.

Doctors at the University of Maryland Medical Center were granted a special dispensation by the US medical regulator to carry out the procedure, on the basis that Mr Bennett would otherwise have died.

He had been deemed ineligible for a human transplant, a decision that is often taken by doctors when the patient is in very poor health.

For the medical team who carried out the transplant, it marks the culmination of years of research and could change lives around the world.

Surgeon Bartley Griffith said the surgery would bring the world "one step closer to solving the organ shortage crisis", the University of Maryland School of Medicine said in a release. That crisis means that 17 people a day in the US die waiting for a transplant, with more than 100,000 reportedly on the waiting list.

The possibility of using animal organs for so-called xenotransplantation to meet the demand has long been considered, and using pig heart valves is already common.

In October 2021, surgeons in New York announced that they had successfully transplanted a pig's kidney into a person. At the time, the operation was the most advanced experiment in the field so far.

However, the recipient on that occasion was brain dead with no hope of recovery.

Mr Bennett, however, is hoping his transplant will allow him to continue with his life. He was bedridden for six weeks leading up to the surgery, and attached to a machine which kept him alive after he was diagnosed with terminal heart disease.

"I look forward to getting out of bed after I recover," he said last week.

On Monday, Mr Bennett was reported to be breathing on his own while being carefully monitored.

But exactly what will happen next is unclear. The pig used in the transplant had been genetically modified to knock out several genes that would have led to the organ being rejected by Mr Bennett's body, the AFP news agency reports.

Mr Griffith said they were proceeding cautiously and carefully monitoring Mr Bennett, while his son David Bennett Jr told the Associated Press that the family were "in the unknown at this point".

But he added: "He realises the magnitude of what was done and he really realises the importance of it."

"We've never done this in a human and I like to think that we, we have given him a better option than what continuing his therapy would have been," Mr Griffith said. "But whether [he will live for] a day, week, month, year, I don't know."