US President Donald Trump has approved the execution by firing squad of former psychiatrist Nidal Malik Hassan, who was convicted of killing 13 people and wounding 30 others in a 2009 shooting at the Fort Hood military base in Texas, the Pentagon announced Monday.

Pentagon spokesman Sean Parnell explained that Secretary of War Pete Hegseth recommended the sentence be implemented, and Trump agreed to have it implemented by the Army's execution division, while the Secretary of the Army would determine the date and place of implementation.

The execution of the sentence would be the first execution carried out by the US military since 1961.

A military jury sentenced Hassan to death in 2013 after convicting him of "carrying out an al-Qaeda-inspired shooting", while the court was told he opposed the US military presence in Iraq and Afghanistan.