The world is at war but not a religious one, Pope Francis said as he travelled to Poland for a five-day trip in celebration of World Youth Day.

The pontiff spoke to reporters on the papal plane amid a cloud of unease and tension after a priest had his throat slit in a terrorist attack in northern France. ISIS claimed the two assailants, shot dead by police, were their "soldiers" after they took five hostages, seriously injuring one as mass was celebrated in the small town of Saint-Etienne-du-Rouvray.

"It's war, we don't have to be afraid to say this," said Pope Francis, as the killing threatened overshadow the entire trip.

But he later insisted he was speaking of "a war of interests, for money, resources. ... I am not speaking of a war of religions. Religions don't want war. The others want war."

After he arrived in Poland, Francis used his first speech of the trip to urge Poland's right-wing government to welcomes migrants and refugees. He spoke from Krakow's Wawel Castle after a meeting with the president, prime minister and other leaders. He said Poland needed "a spirit of readiness to welcome those fleeing from wars and hunger, and solidarity with those deprived of their fundamental rights, including the right to profess one's faith in freedom and safety".

He urged politicians to do "everything possible to alleviate the suffering" and called of Polish people to "favour a climate of respect between all elements of society and constructive debate on differing positions".

He concluded: "Life must always be welcomed and protected. These two things go together – welcome and protection, from conception to natural death. All of us are called to respect life and care for it."

Poland is proud of its strong Catholic heritage and the pontiff will fly into an airport named after Polish Saint John Paul II.

But Pope Francis' outspoken support for refugees puts him at odds with the governing Law and Justice Party in Poland. The right-wing party does not share the pontiff's outlook on refugees nor his outspoken stance against climate change.

Security concerns were high as the Pope landed in Krakow and interior minister Mariusz Blaszczak said more than 39,000 police and other security officers would ensure the safety of the Pope and hundreds of thousands of young people throughout the week-long celebration.

The pontiff will visit the Auschwitz concentration camp on Friday.