Michael Behenna, 35, a former lieutenant in the 101st Airborne Division from Oklahoma, was released from prison in 2014 after serving five years in jail.

Behenna was convicted in 2009 of unpremeditated murder for the death of Ali Mansur, an Iraqi who was suspected of belonging to Al-Qaeda.

Mansur was picked up by US troops following a bomb explosion that killed two members of Behenna's unit but was released.

Behenna later conducted his own interrogation of Mansur and claimed at his trial that he killed him in self-defense.

In granting the pardon, the White House said Behenna's case had "attracted broad support from the military, Oklahoma elected officials, and the public."

Hina Shamsi, director of the National Security Project at the American Civil Liberties Union, condemned the presidential pardon for Behenna.

"This pardon is a presidential endorsement of a murder that violated the military's own code of justice," Shamsi said in a statement.

"The military appeals court found Behenna disobeyed orders, became the aggressor against his prisoner, and had no justification for killing a naked, unarmed Iraqi man in the desert, away from an actual battlefield.

"Trump, as commander-in-chief, and top military leaders should prevent war crimes, not endorse or excuse them," Shamsi said.