Pope Francis spoke up for Muslims at an impromptu airborne press conference, saying it's not true or "just" to equate Islam with violence and terrorism.

He said the inspiration for terrorism isn't Islam but a world economy that worships the "god of money" and drives the desperately poverty-stricken to violence.

The pontiff's own experience in inter-religious dialogue has shown him that Muslims seek peace, he told reporters on a flight back to Rome from Krakow, Poland, where he presided over World Youth Day.

"It is not right and it is not just to say that Islam is terroristic," he said, the Wall Street Journal reported.

In nearly "all religions there is always a small fundamentalist group," Francis added. "We [Catholics] have them, too.

"I don't like to talk about Islamic violence because every day when I look at the papers I see violence in Italy — someone killing his girlfriend, someone killing his mother-in-law. These are baptised Catholics," he said.

"If I speak of Islamic violence, I should speak of Catholic violence. Not all Muslims are violent, not all Catholics are violent," he said. He blamed money as the true crux of the problem.

"I know it is dangerous to say this, but terrorism grows when there is no other option and when money is made a god and it ... is put at the center of the world economy," he said. "That is the first form of terrorism. That is a basic terrorism against all humanity. Let's talk about that."

He talked of marginalised Muslim youth in Europe, adding: "How many youths have we Europeans left empty of ideals? They don't have work, and they turn to drugs and alcohol. They go [abroad] and enroll in fundamentalist groups," the pope said.

Francis had talked of a "world at war" with journalists on his way to Poland, referring to a string of terror attacks and the murder of a Catholic priest in France. But he emphasised again then that it was not a religious war.

"There is war for money," he said then. "There is war for natural resources. There is war for the domination of peoples. Some might think I am speaking of religious war. No. All religions want peace; it is other people who want war."

As he spoke on the plane he briefly sported a hat from Panama, site of the next World Youth Day in 2019, handed to him by a journalist. He was in high spirits despite a painful fall just days earlier while saying mass.

"I was looking at the [image] of the Virgin Mary, and I missed a step," the pope said. "But I feel perfectly fine."

As Francis was heading home several Muslims were attending Catholic mass in churches and cathedrals in France and Italy. It was a gesture of solidarity with Catholics following the gruesome murder of 86-year-old Father Jacques Hamel who had his throat slit in his church in Saint-Etienne-du-Rouvray by two teenage radical Islamists.

"We are very moved by the presence of our Muslim friends, and I believe it is a courageous act that they did by coming to us," said Archbishop Dominique Lebrun after mass at nearby Rouen Cathedral.

Muslim leaders in the town where Hamel was killed have refused to bury the murderer who was killed by police, saying he has "tainted" Islam.

Pope Francis on a plane
Pope Francis tells journalists on a plane as he heads back to Rome that 'not all Muslims are violent.' Reuters/Stefano Rellandini