Alabama has executed an 83-year-old man convicted of a deadly 1989 serial bombing spree, making him the oldest known person put to death in the modern era of US capital punishment.

Walter Moody was put to death by lethal injection at the William C. Holman Correctional Facility in Atmore and gave no final statement, prison officials said. 

It was the eighth execution this year in the United States.

Moody replaced John Nixon, who was 77 when put to death in December 2005 in Mississippi, as the oldest person executed since the US Supreme Court reinstated the death penalty in 1976, according to the Death Penalty Information Centre, which monitors US capital punishment.

Moody was convicted of posting a bomb in 1989 that killed US Circuit Court Judge Robert Vance, 58, and another that killed Georgia civil rights attorney Robert Robinson.

Prosecutors have said Moody sent the bomb to the judge in anger over a 1972 bomb conviction that Moody felt derailed his career and sent another to the civil rights lawyer to confuse investigators.

Moody, who spent more than 20 years on death row, maintained his innocence and the execution was delayed as the US Supreme Court considered last-minute appeals to spare his life, which the court rejected.

Age and poor health were major factors in a botched execution in Alabama earlier this year when the state tried to put to death Doyle Hamm, 61, who had terminal cancer and severely compromised veins.

The execution was called off while Hamm was on a death chamber gurney and medical staff could not place a line for the lethal injection.

Lawyers for Hamm called on the state not to try to execute him again and reached a settlement with Alabama in March that legal sources said would keep him out of the death chamber.

Moody's execution highlighted ageing US death row populations that have led states to put to death 10 inmates aged 70 or older since 2006, including Moody.

Prior to that, there had been none in the modern era of US executions, according to the Death Penalty Information Centre.

More than 40% of US death row inmates are 50 years of age or older, according to the centre.