While most libertarians would argue how freedom of expression is absolute, Pope Francis differs.

For instance, don't expect the pontiff to take an insult to his mother sitting down.

"If my good friend Dr. Gasparri says a curse word against my mother, he can expect a punch," the Pope said, mimicking a punch to Alberto Gasparri, who organizes papal trips.

He had been standing Francis' side during a press conference aboard the papal plane on January 15, Vatican Radio reported.

"It's normal. You cannot provoke. You cannot insult the faith of others. You cannot make fun of the faith of others," said the Pope.

He also stressed that killing in the name of God "is an aberration."

Francis spoke to the media who have accompanied him on his two-country, one-week swing in Asia that started on January 12 in Sri Lanka.

The Pope shared his thoughts about the brutal terror attacks in Paris that hit the satirical magazine Charlie Hebdo while he flew to Manila.

He insisted that free speech is a fundamental human right and a duty of people to speak their minds for common good.

Thousands of people have rallied around Charlie Hebdo, which has frequently prints provocative drawings of the Prophet Muhammad and representatives of other faiths.

Islamic extremists massacred magazine staff on January 8 at its Paris office, and subsequently attacked Jewish people at a kosher supermarket.

Recently, the Vatican and four respected French imams made a declaration against the attacks, but pleaded with media to respect religions.

At the press conference, Francis called an "aberration" any killing made in God's name, maintaining that religion should never be made a reason to justify violence.

Responding to a question on what he thought constituted the boundaries of the freedom of speech, the Pope explained that anything that could offend a person's religious convictions is a limit to free speech.

"There are so many people who speak badly about religions or other religions, who make fun of them, who make a game out of the religions of others," he pointed out.

"They are provocateurs. And what happens to them is what would happen to Dr. Gasparri if he says a curse word against my mother. There is a limit."

"Each person not only has the freedom but also the obligation to say what he thinks in the name of the common good," the Pope noted. But "each religion has its dignity. I cannot make fun of it."