Pope Francis told Vatican administrators on Monday that he intended to press ahead "with firm resolve" to cleanse the Church's bureaucracy which has been rocked by scandals and charges of greed and corruption.

In traditional Christmas greetings to the Curia, the central Vatican administration, the 79-year-old pontiff said he had flu and excused himself for reading the speech seated, though he showed no signs of illness and later stood to greet the dozens of officials.

As it was last year, his speech was dominated by his intentions of ridding the Vatican's bureaucracy of spiritual "diseases" – a reference to alleged corrupt practices among top administrators – despite resistance in the Church hierarchy.

"It seems necessary to state what has been, and ever shall be, the object of sincere reflection and decisive provisions. The reform will move forward with determination, clarity and firm resolve," he said.

At the same gathering last year he issued a stinging criticism of the Curia, listing a catalogue of "diseases" in the bureaucracy including careerism, scheming and greed that had infected them with "spiritual Alzheimer's".

Returning to the theme on Monday, he said: "Some of these diseases became evident in the course of the past year, causing no small pain to the entire body and harming many souls, even with scandals."

This was an apparent reference to a current trial in the Vatican where five people, including two members of the Curia and two journalists, are charged with the theft of confidential papal documents.

The leaks were the basis of two books published last month that depicted a Vatican plagued by greed and corruption and where the pope faces stiff resistance to his reform agenda.

Francis noted cases "of resistance, difficulties and failures" by some Curia employees opposed to his reforms.

Joking that some in the Curia still needed "Curial antibiotics," he added that "diseases and even scandals cannot obscure" the good work that many of them do for the 1.2 billion-member Church.

In a separate address to non-clerical Vatican employees and their families, Francis asked "for forgiveness for the scandals there have been in the Vatican, especially recently" and asked them to pray for those who had "erred".

Shortly after his election two years ago, Francis set out to reform the Italian-dominated Curia, whose power struggles and leaks were widely held responsible for Benedict XVI's decision in 2013 to become the first pope in six centuries to resign.

Francis, the first non-European pope in 1,300 years, has refused many of the trappings of office and made plain his determination to bring the Church hierarchy closer to the poor.

But while he has renounced the spacious papal apartments in the Apostolic Palace, some officials have faced criticism, including former Secretary of State Cardinal Tarcisio Bertone over the restoration of a luxurious retirement apartment.

Bertone, an Italian who was widely blamed for the Curia woes under Pope Benedict, offered at the weekend to repay 150,000 euros ($162,990) to a Church-affiliated children's hospital that helped pay for the costly renovations.