Pope Francis expressed confidence in his treatment Sunday in a message published in place of his weekly Angelus prayer, as the 88-year-old spent a tenth day in hospital.

"I am confidently continuing my hospitalisation at the Gemelli Hospital, carrying on with the necessary treatment -- and rest is also part of the therapy!", he said in the message that Vatican sources said was written in the past few days.

Earlier, the Vatican said the pontiff had a "tranquil" night, was alert and had rested after a respiratory crisis and blood transfusions.

The pope was admitted to Rome's Gemelli hospital on February 14 after experiencing difficulty breathing for several days and subsequently had pneumonia diagnosed in both lungs.

The Vatican described his condition as critical for the first time on Saturday, reporting that the 88-year-old Francis had needed supplemental oxygen and blood transfusions that day after a "prolonged asthma-like respiratory crisis".

"The night was tranquil, the pope rested," the Vatican said in a one-sentence update on Sunday morning without providing further information. (Agencies)