Vatican City - Pope Francis's condition "continues to be critical", the Vatican said on Saturday, saying the 88-year-old was alert but had a respiratory attack that required "high-flow oxygen", and also blood transfusions.

"The Holy Father's condition continues to be critical, therefore, as explained yesterday, the pope is not out of danger," the Vatican said in its regular early evening update.

"This morning Pope Francis presented a prolonged asthmatic respiratory crisis, which also required the application of high-flow oxygen," it said.

Daily blood tests "showed thrombocytopenia, associated with anemia, which required the administration of blood transfusions", it added.

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"The Holy Father continues to be alert and spent the day in an armchair even if he was suffering more than yesterday. At the moment, the prognosis is reserved."

Francis was admitted to Rome's Gemelli hospital on February 14 with bronchitis, but it soon turned into pneumonia in both lungs, causing widespread alarm.

The pontiff's doctors had told a press conference on Friday there was no imminent risk to his life but that he was "not out of danger".

The Vatican earlier confirmed the Argentine pontiff would not deliver his usual weekly Angelus prayer on Sunday, saying the text would be published, as it was last weekend.