Pope Francis is being held in hospital for several days with a lung infection, the Vatican said.
The pontiff, 86, experienced breathing difficulties in recent days, a statement released on Wednesday evening said.
"Pope Francis is touched by the many messages received and expresses his gratitude for the closeness and prayer," the Vatican said.
Hours earlier, it said the Pope had gone to Rome's Gemelli Hospital for a "scheduled check-up".
Earlier in the day he took part in the weekly general audience and appeared to be in good health.
Pope Francis suffers from diverticulitis, a condition that can infect or inflame the colon, and had an operation in 2021 to remove part of the organ.
This year, he said the condition had returned and that it was causing him to put on weight, but that he was not overly concerned. He did not elaborate.
He also has a problem with his knee and alternates between using a cane and a wheelchair in his public appearances.
Pope Francis said last year that he preferred not to have surgery on his knee because he did not want a repeat of long-term side-effects from anaesthesia he suffered after the 2021 operation.
Last July, returning from a trip to Canada, the Pope acknowledged that his advancing age and his difficulty walking might have ushered in a new, slower phase of his papacy.
But since then, he has visited Kazakhstan and Bahrain, and made a gruelling trip last month to the Democratic Republic of the Congo and South Sudan.
Pope Francis also said this year that he has no plans to resign any time soon and that if he eventually did it would be for serious heath reasons, such as if he were incapacitated.
Asked by Italian-Swiss television channel RSI in an interview broadcast on March 12 what condition would lead him to quit, he said: “A tiredness that doesn't let you see things clearly. A lack of clarity, of knowing how to evaluate situations.”
Ten years of Pope Francis - in pictures
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It is 10 years since Pope Francis became head of the Roman Catholic Church. The National looks back at that decade. Getty Images -
Pope Francis attends the funeral mass for Pope Emeritus Benedict XVI in January 2023 at St Peter's square in Vatican City. Getty Images -
Pope Francis leads mass with the new cardinals in August 2022 at Vatican City. Getty Images -
Pope Francis in March 2021 conducts mass at the Franso Hariri Stadium in Erbil, Iraq. Getty Images -
Pope Francis arrives at the Franso Hariri Stadium in Erbil, Iraq, March 2021. Getty Images -
St Peter's Square stands empty while Pope Francis celebrates the Easter Mass in an empty St Peter's Basilica during the coronavirus pandemic in April 2020. Getty Images -
Pope Francis drives around Tokyo Dome before conducting mass during a visit to Japan in November 2019. Getty Images -
Sheikh Mohamed bin Zayed, who was Crown Prince of Abu Dhabi and Deputy Supreme Commander of the UAE Armed Forces at the time, with Sheikh Mohammed bin Rashid, Vice President and Ruler of Dubai and Pope Francis at the Presidential Palace in Abu Dhabi, in February 2019. Photo: Ministry of Presidential Affairs -
Pope Francis celebrates mass at Zayed Sport City, Abu Dhabi, in February 2019. Getty Images -
Pope Francis attends the festival of families at Croke Park in August 2018 in Dublin, Ireland. Getty Images -
Pope Francis during the Stations of the Cross at the Colosseum in March 2018 in Rome. Getty Images -
The Pope at a public engagement in November 2017 in Yangon, Myanmar. Getty Images -
Pope Francis at the Celebration of the Lord's Passion at St Peter's Basilica, Vatican City, April 14, 2017. Getty Images -
Pope Francis at Easter Mass at St Peter's Square, Vatican City, April 2017. Getty Images -
Pope Francis with Prince Charles, Prince of Wales during the British royal's visit to the Vatican in April 2017. Getty Images -
Pope Francis meets migrants at the Moria detention centre in Lesbos, Greece, April 2016. Getty Images -
Father Paschal Mwijage of the St Joseph the Worker Parish in Nairobi's Kangemi slum wears a T-shirt featuring Pope Francis in Kenya in November 2015. Getty Images -
Pope Francis after conducting mass at Tacloban Airport in the Philippines, January 2015. Getty Images -
Pope Francis greets Pope Emeritus Benedict XVI at St Peter's Basilica in September 2014. Getty Images -
Pope Francis meets football legend Diego Maradona during an audience in Rome in September 2014. Getty Images -
Pope Francis prays by the Western Wall in Jerusalem, Israel, May 2014. Getty Images -
Pope Francis welcomes Britain's Queen Elizabeth II for a private audience in Rome, April 2014. Getty Images -
Pope Francis during his weekly audience in St Peter's Square, the Vatican, in March 2014. Getty Images -
Pope Francis during the Inauguration Mass for the Pope in St Peter's Square in March 2013. Getty Images -
Women read a special edition of L'Osservatore Romano newspaper, which carries a photograph of the newly elected Pope Francis, March 2013. Getty Images -
Pope Francis on the central balcony of St Peter's Basilica in March 2013. Getty Images
