Pope Francis complained Wednesday of a knee problem that prevented him from personally greeting the faithful following his weekly general audience.

"I won't be able to come among you today to greet you, because I have a problem with my right leg, a ligament in my knee is inflamed," the 85-year-old pontiff said in a rare public comment on his health.

"It's temporary, I hear it happens to old people, I don't know why it happens to me," he joked, drawing laughter from the audience gathered in the Vatican's Paul VI Audience Hall.

The Argentine pope usually descends a series of stairs from the stage of the hall following the weekly audience to shake hands and meet people in the front row.

Francis suffers from chronic hip pain that makes him limp and in July, he underwent an operation on his colon. The pontiff's health is regularly the subject of rumours within the Vatican, particularly among his critics.

In impromptu remarks during the audience, Francis called on parents never to condemn their children for whatever reason, including over their sexual orientation -- advice he has imparted in the past.

"I think of parents with children with illnesses, even permanent illnesses -- how much pain. Parents who see different sexual orientations in their children -- how to deal with this, how to be on their side and not hide in a condemnatory attitude," he said.

"Parents who see their children not going to school -- so many parents' problems, we think how to help them. To them I say: don't be afraid... Never condemn a child."

He recounted how, in Buenos Aires, he felt moved seeing a line of mothers outside a prison, waiting to see their sons "who had made a mistake".

"They showed their faces, they didn't hide and they supported him, always. What courage."

Francis has frequently defended the rights of gay people and argued they should be accepted in their parishes, while urging parents not to reject their gay children.

He has always, however, voiced opposition to marriage among single-sex couples.