The Vatican on Sunday released the first image of Pope Francis in hospital since he began treatment for double pneumonia, in which the 88-year-old pontiff appeared to be breathing unaided.

The pope was admitted to Rome's Gemelli hospital on February 14 with a severe respiratory infection that has required evolving treatment. He has not been seen in public since.

Francis is pictured from behind, sitting facing the altar in a chapel at the hospital. The side of his face is visible and his right hand rests on his lap. There is no sign that he is receiving the supplementary oxygen that he has been given throughout his stay.

The Vatican said the photo was taken on Sunday, when the pope celebrated Mass with other priests in the chapel.

In its latest medical update, issued on Saturday, the Vatican said Francis was gradually improving and was using less mechanical ventilation at night to help with breathing.

The pope has been described as being in a stable or improving condition for nearly two weeks, but the Vatican has not yet given a timeframe for his discharge, saying his recovery is going slowly.

Francis is prone to lung infections because he had pleurisy as a young adult and had part of one lung removed.

He has been receiving both respiratory physiotherapy to help with his breathing and physical therapy to help with his mobility. He has used a wheelchair in recent years due to knee and back pain.

Francis celebrated the 12th anniversary of his election as pope from hospital on Thursday.

Doctors not involved in the pope's care have said he is likely to face a long, fraught road to recovery, given his age and other medical conditions.

On Saturday, the Vatican announced that Francis had approved a new three-year process to consider reforms for the global Catholic Church, in a sign that he intends to continue as pope, despite his health difficulties.