Pope Francis, in hospital for over a month with pneumonia, did not need to use an oxygen mask overnight, the Vatican said Tuesday, as the 88-year-old's condition gradually improves.

Doctors at Rome's Gemelli hospital have said Francis is now stable, after a critical period marked by breathing crises that raised fears for his life -- though they have yet to say when he might leave hospital.

"There have been some slight improvements" in the pope's condition and he "did not need to use" an oxygen mask overnight, the Vatican said in an evening briefing.

Instead, Francis used a cannula -- a plastic tube tucked into his nostrils that delivers high-flow oxygen.

But the Vatican cautioned that this did not mean the pope will no longer need the mask, which he has worn overnight for most of his stay.

Francis, who had part of one lung removed as a young man and is prone to respiratory illnesses, is "stable" but his "clinical picture remains complex", it said.

The next medical bulletin is expected on Wednesday afternoon.

In hospital, Francis has alternated rest with prayer, as well as bits of work when he can.

That has included penning a letter to Italy's leading newspaper, published Tuesday, in which he called for an end to war and urged the media to "serve the truth".

Emphasising the need for responsible journalism in a time of conflict, the head of the Catholic Church said the media must "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," Francis wrote to the Corriere della Sera, in a letter dated March 14.

"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."

- 'Human fragility' -

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

The letter was written in response to a note sent by Corriere's director Luciano Fontana to the pope, who has been in hospital since February 14.

Francis, who has regularly called for an end to conflicts around the world since his election in 2013, 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, the Argentine pontiff said, "requires commitment, work, silence, words".

Despite Francis's progress, speculation continues that he could step down due to his fragility, following in the footsteps of his predecessor, Benedict XVI.

On Monday, Vatican Secretary of State Pietro Parolin told reporters that he had noted an improvement in Francis' health during a visit last week.

But asked whether the conversation had turned to the pope's resignation, he replied: "No, no, no, absolutely not."

Catholics praying for his speedy recovery have been leaving flowers, candles and notes for Francis outside the hospital.

One of them, Elena Pampaloni, from Tuscany in central Italy, said she had "a very beautiful bond with Pope Francis".

"I was at (St Peter's) Square when he was elected, and I saw the white smoke," she told AFP, referring to the sign that tells Catholics they have a new pope.

"Knowing that he is here at the Gemelli, and in pain, makes me and other pilgrims suffer. I hope he gets better soon", she said.