Pope Francis is being kept in hospital for checks after attending a scheduled appointment at a hospital in Rome, Vatican sources say.
The pontiff's closest staff, including security, are expected to stay the night at the Gemelli Hospital should the Pope not be sent home, a person with direct knowledge told the BBC.
Pope Francis' schedule has reportedly been cancelled for the next two days.
A source said medical checks would continue for as long as necessary.
It is not clear whether a new ailment prompted his visit to hospital. Vatican spokesman Matteo Bruni said the Pope was taken to hospital for "some previously scheduled checks".
But some Italian media report he may have been suffering from respiratory problems and was then taken to hospital by ambulance.
This is the busiest time of the year for Pope Francis, 86, with many events and services scheduled ahead of Easter weekend.
A Palm Sunday Mass is scheduled this weekend, and Holy Week and Easter celebrations next week.
He is also scheduled to visit Hungary at the end of April.
On Wednesday morning, he presided over his weekly general audience in St Peter's Square. Though he looked to be in good spirits, he was seen grimacing as he was helped with getting into his vehicle, AFP news agency reports.
The pontiff has suffered from mobility problems related to his knee in recent months, forcing him to use a wheelchair.
He also underwent surgery to treat a colon problem at the same hospital in Rome in 2021.
In January, the Pope led the funeral of his predecessor Pope Benedict XVI - who was the first pope to step down voluntarily for centuries. He said this was due to ill health.
Pope Francis has previously indicated that he may also wish to follow in Benedict's footsteps in the event that his health deteriorated.