Rights groups and relatives say a prominent Vietnamese political prisoner has died in prison just weeks after being returned to custody from medical parole.

Human Rights Watch and other groups say Truong Van Suong died Monday morning of heart-related ailments at a prison outside Hanoi.

Suong, a former South Vietnamese military officer, had spent 33 years in prison on a life sentence for his role in a failed attempt to overthrow the communist post-war Vietnamese government.

He was sent home on medical parole in July 2010 with severe heart disease, but was returned to prison on August 19. Suong was 68 years old.