Pope Francis said Sunday that Islam could not be equated with terrorism and warned Europe was pushing its young into the hands of extremists.

"It's not true and it's not correct (to say) Islam is terrorism," he told journalists aboard the papal plane during the return journey from a trip to Poland.

"I don't think it is right to equate Islam with violence".

Francis defended his decision not to name Islam when condemning the brutal jihadist murder of a Catholic priest in France in the latest of a string of recent attacks in Europe claimed by the Islamic State (IS) group.

"In almost every religion there is always a small group of fundamentalists. We have them too."

"If I have to talk about Islamic violence I have to talk about Christian violence. Every day in the newspapers I see violence in Italy, someone kills his girlfriend, another kills his mother-in-law, and these are baptised Catholics."

The pontiff was speaking after Muslims attended Catholic mass in churches around France on Sunday in solidarity and sorrow following the murder of the priest, whose throat was slit at the altar of his church.

In an echo of remarks made during his five-day trip to Poland for a Catholic youth festival, Francis said religion was not the driving force behind the violence.

"You can kill with the tongue as well as the knife," he said, in an apparent reference to a rise in populist parties fuelling racism and xenophobia.

He said Europe should look closer to home, saying "terrorism... grows where the God of money is put first" and "where there are no other options".

"How many of our European young have we left empty of ideals, with no work, so they turn to drugs, to alcohol, and sign up with fundamentalist groups?" he asked.