At a court martial in a US base near Baghdad's airport on Sunday, Sergeant Evan Vela was found guilty of murder and making false official statements.

He had admitted shooting the Genei Nasir al-Janabi after he stumbled upon the hiding place of Vela and his colleagues during an operation last May near the town of Iskandariyah, 40km south of Baghdad.

Before the jury broke to determine his sentence, Vela apologised to the court, the army and to one of the sons the man he shot.

"When I came to Iraq, I didn't come to do anything wrong," Vela told the jury, reading from a handwritten statement.

"I failed my standards, your standards and the standards of the army. All I can say is I'm sorry and ask for mercy."

After al-Janabi accidentally discovered their hiding place, Vela's commanding officer ordered him to shoot the man.