Pope Francis's condition showed "a further, slight improvement", the Vatican said, while continuing to maintain the prognosis for the 88-year-old fighting pneumonia in both lungs was "reserved".

"The clinical conditions of the Holy Father in the last 24 hours have shown a further, slight improvement," said the Vatican in its nightly bulletin on the health of the pontiff, who has been in hospital since February 14. "Although a slight improvement is being recorded, the prognosis remains reserved," it said.

The Argentine pope was originally admitted to Rome's Gemelli hospital with breathing difficulties and bronchitis, but his condition subsequently deteriorated, sparking widespread concern among Catholics.

Respiratory attacks he suffered at the weekend that required high levels of oxygen and blood transfusions have not been repeated, according to the Vatican, although he continues to receive oxygen and do breathing exercises.

The Vatican said a Tuesday CT scan showed "normal progression of the pulmonary inflammatory condition" while blood tests on Wednesday confirmed the improvement. It also said that a mild renal insufficiency detected at the weekend "had resolved."

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

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.