The Vatican said on Saturday evening that the 88-year-old pontiff's condition had stabilised, following an "isolated" breathing crisis a day earlier.
"The night was peaceful, the pope is still resting," said a one-line note from the Vatican on Sunday morning that did not provide more details. A full medical update on the pope's condition is expected on Sunday evening.
Francis was admitted to Rome's Gemelli hospital on February 14 with severe respiratory problems that swiftly degenerated into double pneumonia - a serious infection in both lungs that can inflame and scar them, making it difficult to breathe.
The pope suffered a constriction of his respiratory airways on Friday, akin to an asthma attack.
However, in a more upbeat tone on Saturday, the Vatican said the pope did not have a fever and did not show signs of an increased white blood cell count, adding that his blood flow and circulation remained stable.
An elevated white blood cell count often indicates the presence of an active infection or inflammation.
"The Holy Father's clinical condition remained stable," the Vatican said on Saturday, adding that the prognosis was still guarded, meaning he was not yet out of danger.
The Vatican added on Saturday that for a second day running the pope required non-invasive, mechanical ventilation, alternating between this and "long periods of high-flow oxygen therapy"
Francis has experienced several bouts of ill health over the last two years and is prone to lung infections because he had pleurisy as a young adult and had part of one lung removed.
The pope has not been seen in public since entering hospital, his longest absence from view since his papacy started in March 2013, and his doctors have not said how long his treatment might last.
Francis will not lead his usual Sunday prayer with pilgrims for the third week running. The text of the prayer will be published rather than read out by the pontiff.
