Pope Francis, hospitalised for two weeks with pneumonia in both lungs, spent a peaceful night after suffering a breathing crisis, the Vatican said on Saturday.

"The pope spent a peaceful night and is resting," it said in a statement.

The 88-year-old pontiff was put on a ventilator because of an "isolated" breathing crisis on Friday, the Vatican said.

His doctors said it would take the next 24 to 48 hours to see if this breathing trouble has worsened his condition or not, a Vatican source said.

The pontiff is "not out of danger" but was "in good spirits," the source added.

Francis was hospitalised on February 14 with bronchitis, but his condition deteriorated into double pneumonia.

Medical experts have warned that Francis's age and the chronic respiratory disease from which he suffers mean a sustained recovery will take time.

The pope, head of the world's nearly 1.4 billion Catholics, had part of one of his lungs removed as a young man and has suffered increasing health problems in recent years.

He is prone to bronchitis and suffers from hip and knee pain, which has made him reliant on a wheelchair.

Nonetheless, Francis has continued to work from the special hospital suite on the Gemelli's 10th floor.

He also has been doing breathing exercises in between resting and praying, according to the Vatican. (AFP)