Pope Francis issued a stinging new critique of the Vatican's top administration on Thursday, saying "traitors" stood in the way of his reforms and made any change as hard as cleaning Egypt's Sphinx "with a toothbrush".

For the fourth year running, Francis used his annual Christmas greetings to the Roman Catholic Church’s central bureaucracy, or Curia, to lecture the assembled cardinals, bishops and other department heads on the need for change.

"Reforming Rome is like cleaning the Sphinx of Egypt with a toothbrush," he said, quoting a 19th-century Belgian churchman. The phrase did not evoke much laughter when the pope read it in the frescoed Clementina Hall of the Vatican's Apostolic Palace.

Since his election as the first Latin American pope in 2013, Francis has been trying to reform the Italian-dominated Curia to bring the Church’s hierarchy closer to its members, to enact financial reforms and guide it out of scandals that marked the pontificate of his predecessor, former Pope Benedict.

But he has encountered resistance, particularly as some departments have been closed, merged or streamlined.