Pope Francis described on Sunday an "acute and strong" pneumonia that led to his hospitalisation last month, but the 86-year-old pontiff said he was determined to keep travelling.

"I felt a strong pain at the end of the (March 29) audience," the pope told reporters aboard his papal plane on his return to Rome from Budapest, where the pontiff made a three-day visit.

"I did not lose consciousness but I had a high fever" and the doctor "right away took me to the hospital," he said.

Francis called it an "acute and strong pneumonia, in the lower part of the lung".

The pope spent three nights in the Rome hospital in March before being released on April 1 after being treated with antibiotics.

He regularly takes questions from Vatican reporters on the flight home following papal trips.

The Argentine pope, who had part of one of his lungs removed as a young man, has suffered increasing health issues in recent years, and it was his second stay in hospital since 2021.

"The body responded well to the treatment, thank God," added the pope, who was able to attend services in the Holy Week and Easter following his hospitalisation.

Asked about his next trips, Pope Francis confirmed his intention to go to Lisbon at the beginning of August for the World Youth Day.

"I hope to be able to do it," he said.

Following Lisbon are trips scheduled for Marseille and Mongolia.

"The programme keeps me moving," he joked.

The health of Francis, who was made pope in 2013, has been a topic of intense speculation over the past year, while persistent knee pain has forced the octogenarian to use a wheelchair.