Two retired military officers have been sentenced to life in prison for the murder of a Catholic bishop during Argentina's 1976-1983 military dictatorship.

Documents in the trial included two letters from the Vatican archives provided by Argentine-born Pope Francis.

The murdered bishop had written the letters and sent them to Rome just before his death denouncing the military regime's abuses.

Ex-general Luciano Menendez, 87, was found guilty of ordering the murder of Enrique Angelelli, bishop of the northwestern province of La Rioja, in August 1976.

Retired commodore Luis Estrella was also found guilty in the case.

The military regime claimed that Angelelli, then 53, was killed in a car accident.

Also travelling in the car was the bishop's aide, a priest named Esteban Pinto, who survived the accident and filed the lawsuit.

It is the first time that a junta-era official is found guilty in the killing of a high-ranking church cleric.

Menendez had earlier been found guilty in seven cases of human rights abuses and was already serving a life sentence.

Scores of Catholic priests and nuns were "disappeared," tortured and killed during the dictatorship years.

The victims include two French nuns and the bishop of San Carlos Ponce de Leon.

Around 30,000 people, mostly regime opponents, were killed or went missing during the dictatorship years, according to human rights groups.