Pope Francis, speaking of last week's deadly attacks by Islamist militants in Paris, has defended freedom of expression, but said it was wrong to provoke others by insulting their religion and that one could "expect" a reaction to such abuse.

"You can't provoke, you can't insult the faith of others, you can't make fun of faith," he told reporters on Thursday, aboard a plane taking him from Sri Lanka to the Philippines to start the second leg off his Asian tour.

Francis, who has condemned the Paris attacks, was asked about the relationship between freedom of religion and freedom of expression.

"I think both freedom of religion and freedom of expression are both fundamental human rights," he said, adding that he was talking specifically about the Paris killings.

"Everyone has not only the freedom and the right but the obligation to say what he thinks for the common good ... we have the right to have this freedom openly without offending," he said.

To illustrate his point, he turned to an aide and said: "It is true that you must not react violently, but although we are good friends if (he) says a curse word against my mother, he can expect a punch, it's normal.

"You can't make a toy out of the religions of others," he added. "These people provoke and then (something can happen). In freedom of expression there are limits."

Seventeen people, including journalists and police, were killed in three days of violence that began with a shooting attack on the political weekly Charlie Hebdo, known for its satirical attacks on Islam and other religions.

Referring to past religious wars, such as the Crusades sanctioned by the Catholic Church against Islam, the Pope said:

"Let's consider our own history. How many wars of religion have we had? Even we were sinners but you can't kill in the name of God. That is an aberration."

Pope Francis Philippines Benigno Aquino

Pope Francis waves next to Philippines President Benigno Aquino as the pontiff arrived Thursday at Villamor Air Base in Manila for a state and pastoral visit. (Erik De Castro/Reuters)

The Pope was also asked if he felt vulnerable to an assassination attempt or an attack by Islamic extremists.

Earlier this week, the Vatican denied Italian newspaper reports that U.S. and Israeli intelligence officials had informed the Vatican that there could be an imminent attack by Islamist militants.

Francis said he was more worried about others - rather than himself - being hurt in an eventual attack and that he was confident about security measures in the Vatican and during his trips.

"I am in God's hands," he said, joking about having asked God to spare him a painful death.

"Am I afraid? You know that I have a defect, a nice of dose of being careless. If anything should happen to me, I have told the Lord, I ask you only to give me the grace that it doesn't hurt because I am not courageous when confronted with pain. I am very timid," he said.