US President Donald Trump has approved the execution of former US Army psychiatrist Nidal Malik Hasan by firing squad over the 2009 mass shooting at Fort Hood, Texas, which left 13 people dead.

The decision, announced by the Pentagon on Monday, marks the first execution by the US military since 1961.

Pentagon spokesman Sean Parnell said Defence Secretary Pete Hegseth recommended the firing-squad execution, which Trump subsequently approved.

“The Secretary of the Army will set the time and place of the execution,” Parnell said, describing the development as the long-awaited conclusion to Hasan’s case.

Hasan was sentenced to death by a military jury in 2013 after being convicted of carrying out the November 5, 2009 attack at the sprawling military installation.

The shooting also left more than 30 people wounded, most of them soldiers. Hasan was eventually shot and injured by a civilian police officer who responded to the incident, leaving the officer partially paralysed.

During his trial, Hasan represented himself and neither called witnesses nor testified. He also did not contest the charges against him, prompting his court-appointed lawyers to express concerns that he was deliberately seeking the death penalty.

Prosecutors told the military court that Hasan opposed US military operations in Iraq and Afghanistan and had communicated online with Anwar al-Awlaki, an American-born radical cleric linked to Al-Qaeda who was later killed in a US drone strike in Yemen.

Hasan, a US-born Muslim of Palestinian descent, opened fire inside a medical facility at Fort Hood, a major US military base used by troops preparing for deployment to conflict zones.

Twelve of those killed and 30 of the wounded were members of the US military.