Pope Francis, who is recovering from life-threatening pneumonia, made an unscheduled visit to St Peter's Basilica Thursday, his second surprise event in two days after previously meeting King Charles III.

The public appearance, after Wednesday's unscheduled private audience with the king and Queen Camilla, comes as the 88-year-old Catholic leader recovers at the Vatican after five weeks in hospital.

On Thursday afternoon, the pope entered the sprawling basilica in his wheelchair, greeting workers engaged in restoration work and some gathered pilgrims, Vatican News reported.

When a young restorer caught a glimpse of the pope, he beckoned to her to come closer so he could thank her and shake her hand, the ANSA agency reported.

The young woman replied "that she was sorry that her 'hands were cold' but the pope wanted to shake them anyway", the agency reported.

Monsignor Valerio Di Palma, the canon of St Peter's, told Vatican News the pope's appearance sparked "too much emotion". 

"My vision blurred from the tears and I couldn't even take a photo," he said.

Francis then proceeded to the tomb of Pope Pius X to pray, before departing back to the Santa Marta guesthouse, where he resides. 

On Wednesday afternoon, the pope met privately with Charles and Camilla for 20 minutes, despite Buckingham Palace having earlier cancelled a planned official audience due to the pontiff's frail health.

It was the first meeting between Charles, the head of the Protestant Church of England, and the pope since the monarch ascended to the throne in 2022.

The Vatican published a photo of the meeting on Thursday morning, showing the pope clasping the queen's hand, with the king looking on holding a gift box.

Francis offered his congratulations to the royal couple, who celebrated their 20th wedding anniversary Wednesday, the palace and the Vatican said.

During the encounter, the king -- who is receiving treatment for cancer -- and the pope also exchanged well-wishes for each other's health, the Vatican said.

"Their majesties were delighted the Pope was well enough to host them -- and to have had the opportunity to share their best wishes in person," a Buckingham Palace statement added.

Charles, 76, has been suffering from an unnamed cancer for more than a year and less than two weeks ago he was briefly admitted to hospital after experiencing side effects from his treatment.

He was out of action for a matter of days before resuming his official engagements on 1 April.

Francis, who almost died twice during his treatment for double pneumonia, has been in convalescence since his return to the Vatican on 23 March.

Despite being ordered to rest and recover for two months, the Argentine made an unexpected appearance in St Peter's Square last Sunday at the end of a mass.

On Tuesday, the Vatican said that Francis's voice and mobility were improving, raising hopes that he may take part in upcoming Easter celebrations.

He has been using a cannula — a plastic tube tucked into the nostrils — to help him breathe, notably at night, but was not wearing one in the picture released Thursday.