WASHINGTON (AFP) - A former Vietnam war veteran is set to be executed Tuesday in the southern US state Georgia for killing a police officer, officials said, in the country's first execution of 2015.

Barring a stay or a last-minute clemency, Andrew Howard Brannan will be killed by lethal injection at 7:00 pm Tuesday (0800 am Wednesday Singapore time) at the execution chamber in Jackson, Georgia.

Brannan, a former Vietnam war vet diagnosed with post-traumatic stress disorder (PTSD), was convicted in January 2000 for the murder of 22-year-old police officer Kyle Dinkheller. His lawyers do not dispute that Brannan killed the officer, but argue that he should not be executed since he is mentally ill.

Attorneys said Brannan's psychological state was not sufficiently considered by the court in sentencing him.

"Should a 66-year-old Vietnam War veteran with no prior criminal record and who was 100 percent disabled under the (district attorney) standards, both with PTSD and bipolar disorder, at the time of the murder of the deputy sheriff - should that person be executed?," defense lawyer Joe Loveland told NPR Radio.

Georgia's State Board of Pardons and Paroles was considering clemency Monday, the day before the scheduled execution, but did not say when they would give a final decision.

A dashboard police camera recorded Dinkheller's death on January 12, 1998, after the officer stopped Brannan for speeding.

The video shows Brannan dancing and yelling insults at the officer before taunting the officer to kill him, according to court documents.

Brannan then reached into his car and pulled out a gun, opening fire on Dinkheller while saying "die fucker" as the deputy sheriff tried to hide behind his car.

Brannan shot the officer nine times, including once at close range.

Mr Loveland told NPR the tape also shows "erratic and irrational behavior". The Georgia Supreme Court reinstated Brannan's death sentence in 2008 after a lower court judge struck it down. Last June, the US Supreme Court rejected his appeal.

According to prison authorities, Brannan has requested a final meal of three eggs with gravy, biscuits, sausages, waffles with pecans and strawberries, as well as milk, apple juice and decaffeinated coffee.

If executed, Brannan will be the first person killed on death row in the United States this year.

In 2014, the number of executions in the United States fell to its lowest in 20 years, with a total of 35 inmates executed across the country, according to the Death Penalty Information Center.

A total of 29 states plus the US capital Washington have either abolished the death penalty or no longer use it.