Pope Francis said on Saturday that his advancing age and his difficulty walking have ushered in a new, slower phase of his papacy and repeated that he would be ready to resign one day if serious health problems prohibited him from running the Church.

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

For the past few months Francis, 85, has been using a wheelchair, a cane or a walker because of knee pain caused by a fracture and inflamed ligament.

He walked with a cane to the rear cabin where reporters travel, but sat in a wheelchair for the traditional, 45-minute post-trip news conference, the first time he has done so on his 37 international trips since he was elected as pope in 2013.

"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.

The pace of the trip to Canada, which centred on his apologies for the Church's role in residential schools to assimilate indigenous children, was slower than in the past, with usually only two events a day and long patches of rest time.

Francis said he preferred not to have an operation on his knee because he did not want a repeat of long-term negative side effects from anaesthesia he suffered after an intestinal operation a year ago.

"But I will try to continue to travel in order to be close to people because it is a way of serving," he said.

He indicated that he would first make trip to places he had already promised to go, such as South Sudan, Democratic Republic of Congo, Lebanon and perhaps Kazakhstan before deciding on future trips.

"I have all the good will but we will have to see what the leg says," he said. (Reuters)