On his flight back to Rome on Monday after his 10-day visit to the United States and Cuba, Pope Francis weighed in on the issues currently faced by Christians in the United States, including a reference to Kentucky county clerk Kim Davis who has been under fire for refusing to issue same-sex marriage licences.

The Pope said government officials have a "human right" to refuse a duty if it violates their conscience or goes against their faith, a Reuters report said.

"Conscientious objection must enter into every juridical structure because it is a right," he said in Italian. "I can't have in mind all cases that can exist about conscientious objection but, yes, I can say that conscientious objection is a right that is a part of every human right."

"And if someone does not allow others to be a conscientious objector, he denies a right," he pointed out.

Pope Francis also said that conscientious objection should be respected by legal structures. "Otherwise we would end up in a situation where we select what is a right, saying: 'This right has merit, this one does not,'" he said.

Another topic Pope Francis touched upon during his talk with journalists aboard the flight was the issue of priests who are sexually exploiting children. Just the day before, the Pope met with five victims of sexual abuse. He condemned the act emphatically, saying that the victims of religious leaders have been "crushed by evil."

"We know abuses are everywhere, in families, in the neighbourhoods, in the schools, in the gyms, but when a priest abuses it is very grave because the vocation of the priest is to make that boy, that girl grow toward the love of God, toward maturity," he said.

"But instead (the victim) is crushed by evil and this is nearly a sacrilege because the priest has betrayed his vocation, the calling of the Lord," he said.