Updated 1.45pm

Pope Francis has warned that he must slow down his rate of travel or consider resigning, as knee pain and old age complicate his work schedule. 

"I don't think I can go at the same pace as I used to travel," Francis, 85, told reporters on the papal plane as he left Canada. 

"I think that at my age and with this (knee) limitation, I have to save myself a little bit to be able to serve the Church. Or, alternatively, to think about the possibility of stepping aside." 

It was not the first time Francis has said that -- should his health require it -- he could take a page from his predecessor Benedict XVI, who made history in 2013 by stepping down due to declining physical and mental health. 

In 2014, a year into his papacy, Francis told reporters that if his health got in the way of his functions as pope, he would consider stepping down.

In May, as reported in the Italian media, Francis joked about his knee during a closed-door meeting with bishops, saying: "Rather than operate, I'll resign."

"The door is open, it's one of the normal options, but up until now I haven't knocked on this door," he said Saturday.

"But that doesn't mean the day after tomorrow I don't start thinking, right? But right now I honestly don't. 

"Also this trip was a little bit the test. It is true that you cannot make trips in this state, you have to maybe change the style a little bit, decrease, pay off the debts of the trips you still have to make, rearrange.

"But the Lord will tell. The door is open, that is true."

Earlier this month, however, he said he had no plans to emulate Benedict "for the moment" and shot down rumours that he was seriously ill as "gossip". 

Intense speculation

The comments come after intense speculation about Francis's future, after he was forced to cancel a string of events due to his knee pain including a trip to Africa planned for earlier this month.

Talk was also fuelled by his decision to call an extraordinary consistory for August 27, a slow summer month at the Vatican, to create 21 new cardinals -- 16 of whom will be under the age of 80, thereby eligible to elect his successor in a future conclave.

Benedict's decision to quit caused shockwaves through the Catholic Church. He was first pope to resign since the Middle Ages, but the precedent has now been set.

"In all honesty, it is not a catastrophe, it is possible to change pope, it is possible to change, no problem! But I think I have to limit myself a bit with these efforts," Francis said on Saturday.

He mostly used a wheelchair during his trip to Canada, where he offered a historic apology for decades of abuse of Indigenous children at residential schools run by the Catholic Church.

But he did stand up in his "popemobile" to greet crowds.

Francis said surgery on his knee was not an option, adding that he was still feeling the effects of six hours spent under anaesthetic last summer, when he underwent an operation on his colon.

"You don't play, you don't mess around, with anaesthesia," he said. 

But he added: "I will try to continue to go on trips and be close to people, because I think it is a way of service, closeness."

Francis still hopes to reschedule his postponed trip to South Sudan and the Democratic Republic of Congo.

"But it will be next year, because of the rainy season -- let's see: I have all the good will, but let's see what the leg says," he quipped.

The Argentine pontiff repeated that he would like to visit war-torn Ukraine, but offered no details on the state of his plans.

He has another overseas trip planned for a religious congress in Kazakhstan in September.

"For the moment, I would like to go: it's a quiet trip, without so much movement," the pope said.

A podium used by the Pope during his visit to Malta earlier this year was designed with a sacristy beneath it, to spare the pontiff a 300-step walk. 

Canadian 'genocide'

The Pope was speaking at the end of a six-day "penitential pilgrimage" across Canada - his 37th international voyage since becoming pope in 2013. 

During the trip, he offered a historic apology to the First Nations, Metis and Inuit people, who have been waiting for years for such an acknowledgement from the head of the world's 1.3 billion Catholics. 

Aboard the papal plane, he used the word "genocide" to describe the decades of maltreatment and sexual abuse against Canada's Indigenous children, who were wrenched from their families and cultures to attend state schools run by the Church. 

"I didn't say the word (in Canada) because it didn't come to my mind, but I did describe the genocide. And I asked for forgiveness for this process which was genocide," he told reporters.

Although Francis's unprecedented apology was mostly welcomed across Canada, from western Alberta to Quebec and the far north, many survivors said much more needed to be done for reconciliation.

'Evil perpetrated'

Francis wrapped up his journey Friday in the capital of the vast northern territory of Nunavut, Iqaluit, which means "the place of many fish".

There, he asked for forgiveness for "the evil perpetrated" by Catholics in the 139 residential schools across Canada run by the Catholic Church, where about 150,000 Indigenous children were sent from the late 1800s to the 1990s. 

"I want to tell you how very sorry I am and to ask for forgiveness for the evil perpetrated by not a few Catholics who contributed to the policies of cultural assimilation," he said. 

Many were physically and sexually abused at the schools, and thousands are believed to have died of disease, malnutrition or neglect, in what a truth and reconciliation commission later called a "cultural genocide".

Residents in Iqaluit, a community of just over 7,000 people and where small houses line the rocky ocean shore, have listened closely to the pope's words throughout his trip.

"He did apologise, and a lot of people don't seem to be happy with it, but he took that step to come to Nunavut... and I think that's big," lifelong Iqaluit resident Evie Kunuk, 47, told AFP. 

'Brilliant light'

Throughout the trip, Indigenous people have spoken of a "release of emotion" at hearing the pope's words, while warning it was only the beginning. 

Some have called for Francis to rescind the Doctrine of Discovery, the 15th-century papal bulls that allowed European powers to colonise any non-Christian lands and people.

"This doctrine of colonisation, it's true, it's bad, it's unfair, and even today it's used," he told reporters on Saturday, adding that "there has always been a danger, a mentality of 'we are superior and these indigenous people don't matter', and that is serious".

He said it was necessary to "go back and clean up everything that was done wrong, but with the awareness that today there is the same colonialism", he said, citing the case of the Rohingyas in Myanmar.

Demands were also made in Canada for access to records documenting what happened in the schools, and for the Vatican museums to return Indigenous artefacts.

Many have observed that the pope did not specifically mention or apologise for the sexual abuse committed at the schools.

Inuit leaders had been expected to again ask the pope to intervene in the case of 93-year-old Joannes Rivoire, a fugitive French priest accused of sexually abusing Inuit children in Nunavut decades ago.

Earlier this year, Canadian police issued a new arrest warrant for Rivoire, and an Inuit delegation asked Francis to personally intervene to see him extradited.

Francis did not publicly mention Rivoire.

In Iqaluit, he spoke of the "beautiful relationship" between the Inuit and the land.

He said that "it too is strong and resilient, and responds with brilliant light to the darkness that enshrouds it for most of the year."