VATICAN CITY (Reuters) - 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”.

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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.

Francis said some in the bureaucracy - the nerve center of the 1.2-billion-member Church and whose members are entrusted with carrying out the pope’s decisions - were part of “cliques and plots”. Francis called this “unbalanced and degenerate” and a “cancer that leads to a self-referential attitude”.

In his address on Thursday, he spoke of those “traitors of trust” who had been entrusted with carrying out reforms but “let themselves be corrupted by ambition and vainglory.”

When they are quietly let go, he said, “they erroneously declare themselves to be martyrs of the system ... instead of reciting a ‘mea culpa’” (Latin for “my fault”).

Francis did not cite any specific examples.

Last June the Vatican’s first auditor general resigned suddenly. He later said he was forced to step down because he had discovered irregularities but the Vatican said he had been spying on his superiors.

Earlier this month, the Vatican bank’s deputy director was fired under circumstances that have not been explained.

In July, in a major shake-up of the Vatican administration, Francis replaced Catholicism’s top theologian, a conservative German cardinal who has been at odds with the pontiff’s vision of a more inclusive Church.

Francis said the overwhelming majority of Curia members were faithful, competent and some saintly.

Later, in a separate meeting with lay Vatican employees and their families, Francis asked forgiveness for the failings of some Church officials.

He spoke hours before the funeral of Cardinal Bernard Law, the ex-Archbishop of Boston who resigned in disgrace after covering up years of sexual abuse of children by priests and whose name became a byword for scandal in the Catholic Church.