WASHINGTON (AFP) - An American man confessed to a murder he had long denied as he was put to death on Tuesday night in Florida by lethal injection.

William Happ, 51, had spent 24 years on death row in connection with the death of a young woman named Angela Crowley in 1986. The state corrections system used a new lethal chemical for the first time, and the execution reportedly did not go smoothly.

Florida news reports said Happ took more than 15 minutes to die, as his body repeatedly made convulsive movements. He was pronounced dead at 2016 GMT (2am Tuesday Singapore time), said Ms Misty Cash, a spokesman for the state prison system.

Happ, who had drug and alcohol problems, met Ms Crowley in a parking lot. The woman was strangled and raped, and her body found in a canal. He was initially arrested for an unrelated crime, and the murder probe led to him because of a footprint of his that was found by investigators.

In a statement expressing his last words as he lay on the table where he was ultimately executed, Happ confessed.

"For 27 years, the horrible murder of Angela Crowley has been clouded by circumstantial evidence and uncertainty. For the sake of her family, loved ones and all concerned, it is to my agonizing shame that I must confess to this terrible crime," Happ wrote.

He added: "I wish to offer my most sincere and heartfelt apologies not only to those concerned for Angela Crowley but also to those I deceived and allowed to believe in my innocence".

It was the 80th execution in Florida since capital punishment was reinstated in the United States in 1976. It was the 31st this year in the country.

As stocks of the usual lethal injection drug dwindle, most US states that carry out executions are turning to new drugs. This is sparking lawsuits from death row prisoners that the changes will cause undue suffering. Such is the case of another death row inmate in Florida who is scheduled to be put to death next month.