Pope Francis says he could retire from papacy over health concerns

The pope has said he would be ready to resign one day if serious health problems prevented him from running the Catholic Church.

Pope Francis said his advancing age and difficulty walking have ushered in a new, slower phase of his papacy.

"I don't think I can continue doing trips with the same rhythm as before," the 85-year-old told reporters aboard the plane returning to Rome from a week-long trip in Canada.

For the past few months, he has been using a wheelchair, cane, or walker because of knee pain caused by a fracture and inflamed ligament.

"I think that at my age and with this limitation I have to preserve myself a bit in order to be able to serve the Church, or decide to step aside," Francis said.

He has previously said he could follow in the footsteps of Pope Benedict, who in 2013 became the first pope in 600 years to resign instead of rule for life.