The 88-year-old, who has been in hospital for a week with pneumonia and a complex lung infection, also received blood transfusions after tests showed low counts of platelets, which are needed for clotting, associated with anaemia, the Vatican said in a health update.

"The Holy Father's condition continues to be critical, therefore, as explained yesterday (Friday), the pope is not out of danger," the statement said.

It was the first time "critical" was used, in a written statement, to describe Francis' condition.

The statement also said that the pontiff "continues to be alert and spent the day in an armchair although in more pain than yesterday. At the moment the prognosis is reserved".

Doctors have said Francis' condition is touch-and-go and that he is by no means out of danger.

The update, which is drafted by Francis' medical team but issued by the Vatican, also marked the first time the pope's prognosis had been described as "reserved", which suggests it is in flux and requires close observation.

Doctors have warned that the main threat facing Francis would be the onset of sepsis, a serious infection of the blood that can occur as a complication of pneumonia.

As of Friday, there was no evidence of any sepsis, and Francis was responding to the various drugs he is taking, the pope's medical team said in their first in-depth update on the pope's condition.

Francis, who has chronic lung disease, was admitted to the Gemelli hospital on February 14 after a week-long bout of bronchitis worsened.

Doctors first diagnosed the complex viral, bacterial and fungal respiratory tract infection and then the onset of pneumonia in both lungs.

They prescribed "absolute rest" and a combination of cortisone and antibiotics, along with supplemental oxygen when he needs it.

Saturday's update marked the first time the Vatican has referred to Francis suffering an "asthmatic respiratory crisis of prolonged magnitude, which also required the application of oxygen at high flows".

Dr Sergio Alfieri, the head of medicine and surgery at Rome's Gemelli hospital, said on Friday the biggest threat facing Francis was that some of the germs that are currently located in his respiratory system pass into the bloodstream, causing sepsis. Sepsis can lead to organ failure and death.

"Sepsis, with his respiratory problems and his age, would be really difficult to get out of," Dr Alfieri told a news conference on Friday. "The English say 'knock on wood', we say 'touch iron'. Everyone touch what they want," he said as he tapped the microphone. "But this is the real risk in these cases: that these germs pass to the bloodstream."

"He knows he's in danger," Dr Alfieri added. "And he told us to relay that."

Deacons, meanwhile, were gathering at the Vatican for their special Holy Year weekend.

Francis got sick at the start of the Vatican's Holy Year, the once-every-quarter-century celebration of Catholicism. This weekend, Francis was supposed to have celebrated deacons, a ministry in the church that precedes ordination to the priesthood.

In his place, the Holy Year organiser will celebrate Sunday's Mass, the Vatican said.

And for the second consecutive weekend, Francis will skip his traditional Sunday noon blessing, which he could have delivered from hospital if he was up to it.