Pope Francis was "resting" Thursday after spending a "peaceful night", as he nears three weeks in hospital with pneumonia, the Vatican said.

The 88-year-old used an oxygen mask overnight for a third time, the Holy See said. 

The pontiff was hospitalised on February 14 at Rome's Gemelli hospital with breathing difficulties but is now battling pneumonia in both lungs.

Francis missed the formal Ash Wednesday celebrations in Rome marking the start of Lent, but took part in a blessing in the private suite reserved for popes on the 10th floor of the Gemelli.

Christians across the globe celebrating Ash Wednesday prayed for the pontiff's recovery, including in his native Argentina.

Francis, leader of the world's almost 1.4 billion Catholics, has not been seen in public since his hospitalisation, nor has the Vatican issued any photos, although he has published several texts.

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