Pope Francis, who is in critical condition in hospital battling double pneumonia, rested well throughout the night, the Vatican said Tuesday.

The 88-year-old pope was admitted to Rome's Gemelli hospital Feb. 14 with a serious respiratory illness that has since triggered other health problems.

"The Pope rested well all night," the Vatican said in a brief, one-sentence statement.

The Vatican said Monday the pontiff's condition remained critical but had nonetheless shown a "slight improvement," adding the "mild kidney insufficiency," which was first reported at the weekend, was not a cause for concern.

Double pneumonia is a serious infection that can inflame and scar both lungs, making it difficult to breathe. The Vatican has described the pope's infection as "complex," and said it was caused by two or more microorganisms.

Francis, who has been pope since 2013, has suffered bouts of ill health over the past two years. He is particularly prone to lung infections because he developed pleurisy as a young adult and had part of one lung removed.