WASHINGTON, United States (AFP) — An inmate convicted of murdering a man over a US$200 drug debt by burning him alive was executed by nitrogen gas in the southern US state of Alabama on Thursday.

Anthony Boyd, 54, who has steadfastly maintained his innocence, was sentenced to death in 1995 for the murder two years earlier of 32-year-old Gregory Huguley.

He was pronounced dead at 6:33 pm Central time (2333 GMT) at a state prison in the town of Atmore, the state department of corrections announced.

At trial, prosecutors said that Boyd and three other men abducted Huguley at gunpoint because he allegedly failed to pay for US$200 of cocaine.

Huguley was driven to a baseball field, bound with duct tape, doused with gasoline and set on fire.

Boyd was convicted largely on the testimony of a co-defendant, Quintay Cox, who was spared the death penalty.

This was the 40th execution in the United States this year, the most since 2012, when 43 inmates were put to death.

Florida has carried out the most executions with 14, followed by Texas and Alabama with five each.

Nitrogen hypoxia, which involves pumping nitrogen gas into a face mask, causes the prisoner to suffocate.

The use of nitrogen gas as a method of capital punishment has been denounced by United Nations experts as cruel and inhumane.

The death penalty has been abolished in 23 of the 50 US states, while three others — California, Oregon and Pennsylvania — have moratoriums in place.

President Donald Trump is a proponent of capital punishment and, on his first day in office, called for an expansion of its use “for the vilest crimes”.