Pope Francis is under observation Monday after undergoing colon surgery, according to Vatican City officials.

The 84-year-old "responded well" to the treatment, which was performed under general anaesthetic Sunday, Matteo Bruni, Director of the Holy See Press Office, said.

Bruni stunned the faithful throughout the church when he announced the Pope had been taken to the Gemelli hospital in the Italian capital for "scheduled surgery" to treat symptomatic diverticular stenosis of the colon, The New York Times reported.

"Performing the surgery is Prof. Sergio Alfieri. Dr. Alfieri is in the hospital's Department of Medical and Surgical Sciences and heads the Digestive Surgery Complex Operational Unit," People quoted the Vatican representative as saying.

Alfieri specializes in general, digestive, colon-rectal, stomach, and pancreatic surgery, Bruni said.

Diverticular stenosis of the colon causes a painful inflammation of the diverticulum, a sac that can form on the colon walls and which tend to grow with age, the American Society of Colon and Rectal Surgeons said.

It is the first time Francis has been admitted to hospital since becoming pope in 2013. The Vatican did not disclose further details about the Pope's surgery or how long he would remain in hospital.

Some 10 doctors, including Alfieri, and Francis' personal physician, Dr. Roberto Bernabei, were present during his surgery, Bruni said.

Francis opened up about his near brush with death from the flu when he was 21 years old, in his 2020 book Let Us Dream: The Path to a Better Future, about navigating the Covid-19 pandemic, according to People.

"I have some sense of how people with coronavirus feel as they struggle to breathe on ventilators," Francis wrote.

The Pope also suffers from a hip problem and sciatica which causes pain that radiates from the lower back to the legs.