PHILADELPHIA (Reuters) - A convicted ax murderer, on Delaware's death row for some 18 years, was executed on Friday at the state prison in Smyrna.

Robert W. Jackson III, 38, was put to death by lethal injection shortly after midnight, Delaware Governor Jack Markell announced.

He was sentenced to death for killing Elizabeth Girardi, 47, with blows from an ax he wielded during a 1992 burglary attempt at her home in Hockessin, Delaware.

It was Delaware's first execution since 2005.

Governor Markell denied a last-minute request for a reprieve, noting that the U.S. Third Circuit Court of Appeals and the U.S. Supreme Court had already decided not to stay the execution.

"Mr. Jackson's death sentence was recommended by a jury, imposed by a judge, and reviewed by state and federal appellate courts at all levels," the governor said in a statement issued at 12:12 a.m. local time, shortly after the death of Jackson, who was 18 at the time of the murder.

"It is my prayer that his victim rests in peace and her family finds some closure, May God have mercy on Mr. Jackson," Markell said.

It was the 30th execution in the U.S. this year compared with 46 in all of 2010, according to the Death Penalty Information Center.

(Editing by Barbara Goldberg and Greg McCune)