WASHINGTON (AFP) - A United States man was executed on Thursday in Florida, where he was convicted of beating two female co-workers with hammers before burning them alive.

Robert Henry, 55, died by lethal injection at 6.16pm after 25 years on death row, the Florida Department of Corrections said. Henry was sentenced to death for the November 1987 murder of Ms Phyllis Harris, 53, and Ms Janet Thermidor, 35, in the fabric store where the three worked in Deerfield Beach.

The African-American maintenance man had told Ms Harris that burglars had instructed him to tie and blindfold her in the restroom, and she complied. He then beat Ms Thermidor with a hammer in the store office, stole cash from the register, doused the area with flammable liquid and set it on fire.

Henry then returned to Ms Harris, beating her with a hammer and setting her ablaze.

Ms Harris was found dead, but the authorities responding to the fire found Ms Thermidor still alive.

Ms Thermidor died the next day, but was able to first blame Henry for the incident.

When first arrested, Henry initially denied the accusations before confessing, court documents showed.

Florida, like other US states, has experienced a shortage of the most commonly used lethal injection anesthetic, pentobarbital.

In response, it has been using a new cocktail since October: midazolam to render the inmate unconscious, followed by two additional drugs to paralyze and kill.

Local press has said the drug mix provoked outbursts and incoherent body movements the first time it was used nearly half a year ago. Henry's was the fifth Florida execution to use the mixture.

His final appeal to the US Supreme Court was rejected earlier by Justice Clarence Thomas, who is in charge of Florida.

It was the 12th US execution this year and fourth in Florida, which has taken steps to accelerate the pace of executions despite controversy over the anesthetic it uses.