Washington (AFP) - A Vietnam war vet with post-traumatic stress disorder was executed Tuesday in the southern state of Georgia for murdering a police officer, the country's first death row inmate killed this year.

Andrew Howard Brannan, 66, was declared dead by lethal injection at 8:33 pm (0133 GMT Wednesday) at an execution chamber in Jackson, Georgia, prison spokeswoman Susan Megahee told AFP.

Brannan, who fought in the Vietnam War, was diagnosed with post-traumatic stress disorder and was convicted in January 2000 for the murder of 22-year-old police officer Kyle Dinkheller.

Georgia's State Board of Pardons and Paroles had considered clemency Monday, but denied a stay of execution and said Brannan was guilty of "malice murder."

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, shooting the officer nine times, including once at close range.

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.

The top US court rejected a last-ditch appeal Tuesday evening.

Brannan's lawyers argued he was a courageous soldier who volunteered to fight for his country and did not deserve to be killed.

"Andrew Brannan was decorated for heroic combat service," said attorney Brian Kammer.

"Brannan suffered severe, debilitating mental illness as a result of his combat experiences whose symptoms continued to manifest up to the time of the crime," Kammer wrote in a final appeal.

The lawyer called for a "categorical exemption" from capital punishment for veterans such as Brannan and argued the execution was unconstitutional.

"A categorical exclusion is warranted because combat veterans with PTSD are meaningfully distinguishable from other capital offenders," he said.

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Brannan's lawyers had not denied that he killed Dinkheller, but argued for clemency based on his mental illness.

"There was a direct connection between his service in Vietnam and the violence that he was exposed to there and the ultimate events that occurred here," defense lawyer Joe Loveland said.

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.