Pope Francis in 'critical condition' after 'prolonged asthma-like ...

Pope Francis's health has deteriorated over the past 24 hours, the Vatican says, adding that he had suffered a "prolonged asthma-like respiratory crisis" and had needed blood transfusions.

"The Holy Father's condition remains critical. The Pope is not out of danger," the Vatican said.

"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."

What we know about the pope's 'complex' respiratory infection

Officials say the 88-year-old pontiff has a "polymicrobial respiratory tract infection", but little else is known about the illness.

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

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

He has since been diagnosed with double pneumonia, a serious infection that can inflame and scar both lungs, making it difficult to breathe.

Sergio Alfieri, a senior member of the Gemelli staff, said there was a risk the lung infection could spread to his bloodstream and develop into sepsis, which "could be very difficult to overcome".

The Vatican has previously described the pope's infection as "complex", saying it is being caused by two or more micro-organisms.

Pope vulnerable due to age and fragility

In a briefing on Friday, two of his doctors said Francis, who is 88 and has served as pope since 2013, was highly vulnerable due to his age and frailty.

Concerns about his health led the Vatican to announce earlier on Saturday that he would not appear in public to lead prayer with pilgrims on Sunday, the second consecutive week he will have missed the event.

The pontiff's stay in hospital is the latest in a series of bouts of ill health in the past two years. He is particularly prone to lung infections because he developed pleurisy as a young adult and had part of one lung removed.

Concerns over his health have led at least one cardinal to suggest he might resign if he felt unable to carry on.

However, Cardinal Pietro Parolin, the Vatican's secretary of state — sometimes referred to as the "deputy pope", dismissed such talk as "useless speculation".

"Now we are thinking about the Holy Father's health, his recovery, and his return to the Vatican. These are the only things that matter," Parolin told Italian newspaper the Corriere della Sera in comments published on Saturday.

Reuters/ABC