Pope Francis has no intention of resigning as he feels that his health is good enough to allow him to carry on, he says in a new book whose excerpts were published by Italy's Corriere della Sera newspaper on Thursday.

"This is a distant hypothesis, because I don't have reasons serious enough to make me think about giving up," Francis was quoted as saying in "Life: My Story Through History," a book due out in Italian and English on March 19.

Francis is 87 and has been increasingly frail in recent years, using a wheelchair or a cane to move around and recently suffering from what have been described as bouts of bronchitis or colds that have led him to limit his public speaking.

Nevertheless, in the book he reassures about his condition.

"Thank the Lord, I enjoy good health and, God willing, there are many projects still to be realized," he said, repeating that he would consider quitting only in case of a "serious physical impediment."

The pope again defended his recent decision to allow priests to bless same-sex couples, saying it is the duty of the Catholic Church to welcome all and that "God loves everyone, especially sinners."

At the same time, the decision does not imply any change in Catholic doctrine - which only recognises heterosexual marriages - and if some bishops do not want to perform same-sex blessings this will not lead to a schism, Francis added.

Elsewhere in the book, he renewed his condemnation of abortion and surrogate parenting, and noted that his focus on the poor and marginalised does not make him a communist or a Marxist.

Francis' predecessor Benedict XVI was the first pope to resign in around 600 years, citing the strains of old age. He quit in Feb. 2013, aged 85, and went on to live for almost 10 more years, dying at the age of 95.