The conflicts afflicting the world are about wealth, power and control of natural resources, not religion, Pope Francis said on Wednesday.

"It's warfare over interests, money and natural resources, I am not speaking of a war of religion," Francis said after knifemen beheaded an 86-year-old priest at a church in northern France on Tuesday in an attack claimed by the Islamic State jihadist group.

"Religions don't want war. The others want war," he added.

He made the remarks to reporters during a flight from Rome to Krakow, where he was due to kick off a five-day visit to Poland by attending the city's World Youth day celebrations taking place till Sunday.

"We shouldn't be afraid of telling the truth: the world is at war because it has lost the peace," Francis added.

Poland's Interior Minister says more than 39,000 police and other security officers will be ensuring security as Francis meets hundreds of thousands of young pilgrims from around the globe in southern Poland.

The papal visit includes open-air masses and prayers with around 1.5 million World Youth Day participant, visits to the site of the former Nazi death camp of Auschwitz-Birkenau and to Jasna Gora, Poland's holiest shrine.

It is the first visit by Francis to Poland, one of Europe's most Catholic nations, which produced late Polish pope, St. John Paul II.