Pope Francis on Friday criticised conservative clergy and bishops who he said had defamed slain Salvadoran Archbishop Oscar Romero even after he was killed by a right-wing death squad in 1980.

The pope departed from his prepared address to a group of visiting Salvadorans to deliver unusually pointed remarks about the past detractors of Romero, who was beatified last May in El Salvador, putting him a step away from sainthood.

"His martyrdom continued (even after his death). He was defamed, slandered...even by his own brothers in the priesthood and the episcopate," Francis said.

Francis said Romero, who was shot while saying Mass in a hospital chapel, had been attacked even after his death by "the hardest stone that exists in the world: the tongue".

Romero, whose defence of the poor made him an icon for many Roman Catholics in Latin America, was beatified as a martyr for the faith.

Francis, the first Latin American pope, unblocked Romero's sainthood process shortly after his election in March 2013.

It had been stalled under popes John Paul II and Benedict XVI because conservative Latin American Church leaders saw Romero as having been too close to Liberation Theology, a radical movement that emphasised helping the poor and opposing injustice.

The conservatives had accused Romero, who spoke out against the Salvadoran government and often denounced repression and poverty in his homilies, of having been an advocate of a Marxist-style class struggle.

They asserted that he was killed for his political views and not for his faith.

The murder was one of the most shocking of the long conflict between a series of US-backed governments and leftist rebels in which thousands were killed by right-wing and military death squads. No one was ever brought to justice for Romero's killing.

The civil war, one of the Cold War's most brutal conflicts, claimed some 75,000 lives before it ended with a peace agreement in 1992.