Pope Francis, who has been battling pneumonia and bronchitis for more than three weeks, has shown a "good response" to treatment, the Vatican has said.

"The clinical condition of the Holy Father in recent days has remained stable and, consequently, indicates a good response to the treatment," the Vatican said.

"There is therefore a gradual, slight improvement", it added.

The 88-year-old prayed on Saturday morning in the chapel of the papal suite at Rome's Gemelli Hospital, where he is being treated.

The Argentine pontiff has not been seen in public since entering the hospital, his longest stretch away from the public eye since his papacy began 12 years ago.

He was admitted to hospital on 14 February after experiencing breathing difficulties - and first treated for bronchitis before being diagnosed with pneumonia in both lungs.

He is particularly vulnerable to pneumonia, an infection of the lungs that can be caused by bacteria, viruses or fungi, after he contracted pleurisy – an inflammation of the lungs – as a young man and had a partial lung removal.

The Pope's doctors believe he is likely to face a long road to recovery, due to his age and medical history.