The US state of Alabama on Thursday executed a man found guilty of murdering of three police officers, despite a campaign to save him by activists including celebrity Kim Kardashian.

Nathaniel Woods, 44, was accused of being the "mastermind" behind the 2004 deaths of police officers allegedly lured into an ambush when they tried to arrest him on drug charges.

Woods did not pull the trigger but was given the same sentence as the gunman, Kerry Spencer.

Spencer described Woods as "100 percent" innocent in a recent letter to US media, saying that "I know this to be a fact because I'm the person that shot and killed all three of the officers."

Woods, who always protested his innocence, died by lethal injection.

"Tonight, justice has been served," Alabama Attorney General Steve Marshall said in a statement.

Woods was convicted by 10 of the 12 jurors in 2005. Alabama is the only US state that does not require a unanimous verdict to impose the death penalty.

Kardashian, a regular campaigner against the death penalty, tweeted earlier that Woods was "scheduled to be executed in Alabama TONIGHT for murders he did NOT commit."

She joined about 120,000 people who had urged Alabama Governor Kay Ivey to grant a reprieve.

"Are you willing to allow a potentially innocent man to be executed?" Martin Luther King III, son of the civil rights leader, asked in a letter to the governor posted on Twitter.

Hours before the execution, a temporary stay was issued on the execution order, but it was then denied by the Supreme Court.

Woods was the fifth person to die by execution in 2020 in the United States.