A critically ill Pope Francis, battling pneumonia in both lungs, was up and out of bed Wednesday after a peaceful night, a Vatican source said.

Francis, 88, was admitted to Rome's Gemelli hospital on 14 February with breathing difficulties, after which his condition deteriorated, sparking widespread concern among Catholics.

After suffering respiratory attacks at the weekend that required high levels of oxygen and blood transfusions, the Vatican has said the pope's condition has stabilised, but he remains in a critical condition.

The pontiff, now in his 13th day at the hospital, was Wednesday "sitting in a chair, and continuing with the treatment as normal," a Vatican source said.

A brief official update from the Holy See earlier said he had "passed a peaceful night and is resting."

As Catholics across the globe pray for Francis, doctors have warned the path to recovery could be long, with the pope likely staying in hospital beyond this week.

A medical bulletin on Tuesday evening said the Argentine pontiff's "clinical condition remains critical but stable."

"There have been no acute respiratory episodes and haemodynamic parameters continue to be stable," the Vatican said, referring to measurements such as heart rate and blood pressure.

Francis had a new scan "in the evening for radiological monitoring of bilateral pneumonia," the Vatican said. "The prognosis remains reserved," it added.

Despite his critical condition, the leader of the world's nearly 1.4 billion Catholics has kept up with Church matters from his hospital suite, according to the Vatican.

On Wednesday he signed off a series of bishop appointments, it said. 

Pilgrims and faithful have been flocking to the Vatican and the Gemelli to pray for Francis, and there will be a fresh round of prayers held in St. Peter's Square on Wednesday evening.