Pope Francis has condemned the habit of linking Islam with terrorism, saying that "nearly all religions" have a "small group of fundamentalists".

Reporters aboard the Catholic leader's plane flying him back to Rome on Sunday after a pilgrimage to Poland, asked him why he never uses the world "Islam" to describe terrorism or other violence.

"It's not right to identify Islam with violence. It's not right and it's not true," he replied.

The pope was in Poland from July 27 until July 31 for World Youth Day, a week-long event attended by more than a million pilgrims.

A day before he left, an elderly Catholic priest was killed in Northern France during Mass, in an attack that was claimed by the Islamic State of Iraq and the Levant.

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"I don't like to talk of Islamic violence because every day, when I go through the newspapers, I see violence," the pope said, in apparent reference to news of crime in the predominantly Catholic country of Italy.

"And these are baptised Catholics. If I speak of Islamic violence, then I have to speak of Catholic violence."

Noting he has spoken with imams, he concluded: "I know how they think, they are looking for peace."

As for the Islamic State of Iraq and the Levant group, he said it "presents itself with a violent identity card, but that's not Islam".