Pontiff suffered "respiratory crisis" and has received blood transfusions as the Vatican warns that Francis is "not out of danger".

Pope Francis remains in a "critical" condition after suffering a "prolonged asthma-like respiratory crisis" that required "high-flow" oxygen, the Vatican said late on Saturday.

The 88-year-old pontiff, who has been hospitalised in Rome for the last week with double pneumonia and a complex lung infection, also received blood transfusions after tests on Saturday revealed "thrombocytopenia, associated with anemia", the Vatican statement said.

Although alert and sitting in his armchair, the pontiff is "more unwell than yesterday" and is "not out of danger", the Holy See said, stressing: "At the moment, the prognosis remains guarded."

The pope was admitted to the Gemelli hospital on 14 February after experiencing bronchitis and difficulties breathing for several days.

The Vatican also confirmed that for the second week in a row Francis would miss the Sunday Angelus which is to be delivered in the form of a written text.

Photo credit: Riccardo De Luca - Update / Shutterstock.com.