Pope Francis on Tuesday called for an end to war and urged reflection in a letter published by an Italian newspaper, as the 88-year-old pontiff recovers from pneumonia in hospital.

In his letter dated 14 March published in the Corriere della Sera , Pope Francis emphasised the need for responsible journalism, for calm minds and noted that the media had a duty to "feel the full importance of words".

"They are never just words: they are facts that build human environments. They can connect or divide, serve the truth or use it," he wrote in the letter published on the front page of the newspaper. 

"We must disarm words, to disarm minds and disarm the Earth. There is a great need for reflection, for calmness, for a sense of complexity."

"While war only devastates communities and the environment, without offering solutions to conflicts, diplomacy and international organisations need new life and credibility," he wrote.

Thanking Corriere's director Luciano Fontana, to whom the letter was addressed, Francis noted that "in this moment of illness... war appears even more absurd".

"Human fragility, in fact, has the power to make us more clear about what lasts and what passes, what makes us live and what kills," he wrote.

Peace, he said, "requires commitment, work, silence, words".

The fourth and longest hospitalisation of Francis's 12-year papacy has seen him confined since February 14 to a suite on the 10th floor of Rome's Gemelli hospital. 

Doctors have said his condition is now stable, after a critical period marked by breathing crises that raised fears for his life. 

On Monday evening, the Vatican said he was now spending short periods breathing on his own.

For at least two weeks, Francis has been alternating an oxygen mask at night with a cannula -- a plastic tube tucked into his nostrils that delivers high-flow oxygen -- during the day.

He is now shifting to a reduced flow for the first time, it said. 

In the hospital, Francis has continued to work when able, while alternating rest with prayer.