A critically ill Pope Francis, battling pneumonia in both lungs, passed a peaceful night, the Vatican said in its latest update Wednesday on the 88-year-old pontiff.

"The pope passed a peaceful night and is resting," the Vatican said in a morning update on the 13th day of Francis's hospital stay.

Francis was admitted to Rome's Gemelli hospital on February 14 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. 

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 hemodynamic 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.

"In the morning, after receiving the Eucharist, he resumed work activities," the statement said.

Pilgrims and faithful have been flocking to the Vatican and the Gemelli to pray for Francis.

Doctors have cautioned that any recovery will take time and that Francis will likely stay in hospital beyond this week.