Pontiff faces several days in hospital.
Pope Francis has a respiratory infection and will need to spend "a few days" in hospital, the Vatican confirmed on Wednesday evening.
The pontiff was admitted to Rome's Gemelli hospital, where he underwent colon surgery two years ago, following his weekly general audience in St Peter's Square on Wednesday morning.
Pope Francis, 86, underwent a CT scan on his chest and abdomen to rule out bronchial complications, the results of which were negative, reports news agency ANSA, citing medical sources.
"In recent days Pope Francis has complained of some respiratory difficulties and this afternoon he went to the Gemelli Hospital to undergo some medical tests," the Vatican said in a statement, ruling out a covid infection.
The Vatican said that the pope, who had part of one lung removed in his 20s, was "touched by the many messages he's received and expresses his gratitude for the closeness and prayer."
All of the pontiff's appointments for the next couple of days have been cleared and his illness comes ahead of a busy Holy Week schedule leading up to Easter next weekend.
Francis is being treated in a section of the Gemelli reserved exclusively for popes, in the same ward that hosted Pope John Paul II after he was shot in 1981.
The pope, who has used a wheelchair since last year due to mobility problems, was seen grimacing as he was helped getting into his vehicle during the audience on Wednesday morning.
In January Francis led the funeral of his predecessor Pope Benedict XVI, who stunned the world 10 years ago by standing down, citing health reasons.
Francis has indicated on several occasions that he may also wish to resign should he become impaired for medical reasons.
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