The Vatican is preparing to open formal debate on opening up the priesthood to married men.
Pope Francis says he has no intention of removing a Roman Catholic rule which requires priests to abstain from marriage and sex.
But the pontiff also said he remained open to investigating the possibility of ordaining older, well-respected married men in remote locations where clergy are in short supply.
Speaking to Reuters and AFP reporters, he said: "Personally, I think that celibacy is a gift to the Church. I am not in agreement (with those who say) optional celibacy should be permitted."
The Catholic leader spoke on-board the papal plane as he returned to the Vatican from Panama, where he had been attending a gathering of young Catholics - World Youth Day.
Campaigners including members of the Movement for Married Clergy have argued that rules on celibacy have been deterring men from joining the priesthood.
Clergy in the Eastern Catholic, Orthodox and Anglican churches are allowed to marry.
In 2017, Premier reported how Pope Francis requested a debate over whether married men should be allowed to serve as priests in Brazil's Amazon region.
There is no formal doctrinal barrier to married men becoming priests, meaning, theoretically, the rules could be changed.
An inquiry into child sexual abuse in Australia recommended in 2017 that the Catholic Church consider introducing voluntary celibacy for clergy.
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