Pope Francis, who is spending his sixth day in hospital for treatment of a respiratory infection, is alert and ate breakfast on Wednesday, the Vatican said in its latest update on the pontiff's fragile health.

Francis has the onset of double pneumonia, the Vatican said on Tuesday, complicating treatment for the 88-year-old pope who was admitted to Rome's Gemelli hospital on February 14.

Double pneumonia is a serious infection that can inflame and scar both lungs and makes breathing more difficult.

The Vatican had said previously that the pope had a polymicrobial infection, which occurs when two or more micro-organisms are involved, adding that he would stay in hospital as long as necessary to tackle a "complex clinical situation".

A Vatican official, who did not wish to be named because he was not authorised to speak about the pope's condition, said on Wednesday Francis was not on a ventilator and was breathing on his own.

The official said the pope had been able to get out of bed and sit in an armchair in his hospital room, and was continuing to do some work.

The Vatican is expected to give a further update on the pope's condition later on Wednesday.

The pope has been plagued by ill health in recent years, including regular bouts of flu, sciatica nerve pain and an abdominal hernia that required surgery in 2023. As a young adult he developed pleurisy and had part of one lung removed.

All the pope's public engagements have been cancelled through Sunday and he has no further official events on the Vatican's published calendar.

Rev. Dr. Andrea Vicini, a Jesuit priest and medical doctor, said it was notable that the Vatican's statement on Tuesday referred to the pontiff as having the onset of pneumonia and not bronchopneumonia. The latter would indicate an infection that is more widespread, he said.

"It (sounds like) it's more localized and has not spread," said Vicini, a professor at Boston College, who said he did not have details of the pope's case beyond the Vatican's public statements.

"If they identified the pathogen, as I expect they would have done, they will have a very targeted therapy," he said. "I am optimistic. It seems they are controlling what is happening."