In a statement on Saturday, the Vatican said the 88-year-old Francis, who has double pneumonia, had suffered a "prolonged asthma-like respiratory crisis" and his condition had worsened in the past 24 hours. 

"The Holy Father's condition remains critical," the statement said. 

"The Pope is not out of danger."

"The Holy Father remains alert and has spent the day in a chair, though he is more unwell than yesterday. At the moment, the prognosis remains guarded."

The Pope was admitted to Rome's Gemelli hospital on February 14 after experiencing difficulty breathing for several days.

The Vatican said on Saturday that doctors had to administer a "high flow" of oxygen because of his breathing crisis. It added that the blood transfusions were necessary because tests showed he had a low platelet count, associated with anaemia.

The Vatican announced earlier in the day that he would not appear in public on Sunday to lead prayer with pilgrims, the second consecutive week he will have missed the event.

Double pneumonia is a serious infection that can inflame and scar both lungs, making it difficult to breathe. The Vatican has described the pope's infection as "complex," saying it is being caused by two or more micro-organisms.

In a briefing on Friday, two of his doctors said the Pope was highly vulnerable due to his age and frailty.

With AAP