Former Pope Benedict publicly endorsed Pope Francis and his ministry in a Vatican ceremony celebrating the 65th anniversary of his ordination to the priesthood on Tuesday.

Benedict XVI, an 89-year-old German, stood without a cane for part of a ceremony in the Clementine Hall of the Apostolic Palace.

But Benedict, in his first public comments in the presence of his successor, did appear to have difficulty pronouncing a few words as he thanked Francis and a small group of cardinals in Italian.

"Thank you, Holy Fathers, for your goodness, which from the first moment of your election has struck every day of my life," said Benedict.

"We hope that you can go forward with all of us on this path of divine mercy, showing us the path of Jesus toward God."

In 2013 Benedict, citing health reasons, became the first pope to resign in some 600 years, ending a papacy of nearly eight years marked by a scandal centred around leaked documents that alleged corruption and mismanagement in the Vatican.

He has since been living in relative isolation in a convent in the Vatican gardens and has made only a handful of brief public appearances, usually at major Church ceremonies together with Francis such as the investiture of new cardinals.

On Sunday, speaking to reporters aboard his plane while returning from a visit to Armenia, Francis was asked about reports that Benedict was a sort of parallel pope, still exercising influence.

"There is only one pope," Francis said. He praised Benedict for "protecting me, having my back, with his prayers".

Francis, who has compared Benedict's presence in the Vatican to having a "wise old grandfather" at home, said he had heard that when some Church officials had gone to Benedict to complain that Francis was too liberal, Benedict "sent them packing".