A key prosecution witness at the court martial of Capt. Robert Semrau testified Wednesday that Semrau claimed he shot and killed a wounded Taliban fighter in a "mercy killing."

Semrau, 36, is facing a second-degree murder charge in connection with the shooting death of an unarmed Taliban prisoner in Afghanistan in October 2008.

The witness, Cpl. Steven Fournier, and Semrau were fire-team partners on the battlefield, meaning they were never more than a few metres apart.

Fournier testified Tuesday that he was only a short distance from Semrau when the captain stood over the broken body of the wounded Afghan man and fired two shots in quick succession.

Fournier testified he didn't see the shots but heard them and quickly looked back to see Semrau closing the ejection port on his rifle — an action soldiers take after they fire their weapon.

On Wednesday, Fournier said that when he heard the shots he wheeled about quickly thinking he was under attack. He said he saw Semrau standing over the insurgent.

Fournier said he was confused and couldn't figure out what had just happened.

He told the hearing that Semrau then said: "It's OK. It was me."

As the two men marched on to join the Afghan unit they were mentoring, Semrau spoke again, Fournier said.

Semrau said he thought the shooting was necessary because it was the humane thing to do, Fournier testified. Semrau said couldn't live with himself if he just left a wounded man to die on the battlefield, the corporal said.

Fournier said Semrau told him that he was willing to accept the fallout from his action and that it was a "mercy-killing."