Pope Francis condemned the failure of the "great powers of the world" to prevent the slaughter of thousands when they knew that German Nazi forces were moving Jews, Christians, homosexuals and others to death camps in Poland during World War II.

The leader of the world's 1.2-billion Roman Catholics issued the strongly worded statement during his visit to Turin in northern Italy on Sunday.

"The great powers had photographs of the railway routes that the trains took to Auschwitz to kill Jews, Christians, homosexuals, everybody," said the pope, referring to the death camp in Poland. He asked: "Why didn't they bomb" those railroad routes?

He also criticised the great powers for dividing Europe "like a cake" after Germany surrendered in May 1945.

The pope also denounced the deaths of Christians in gulags in post-World War II Russia under the Stalin dictatorship.

The pontiff also cited the "great tragedy of Armenia," saying the great powers likewise allowed it to happen.

"In the last century, so many millions (of Armenians) died. But where were the great powers then? They were looking the other way," he said.

The pope drew the ire of Turkey in April when he described the killing of Armenians by Turkish Ottomans as "genocide."

During his impromptu speech, the pope told young people that he understands how they find it difficult to trust the world.

"Everything is done for money," he said to young people.

The Pope on Sunday also told labourers not to blame immigrants for their problems on loss of work opportunities, condemning instead the world's economic system and what he termed the "idolatry of money," the National Catholic Reporter wrote.

"It makes you cry to see the spectacle in these days of human beings treated like merchandise," he said, referring to the continuing crisis of migrants from northern Africa and Middle East entering Italy.

"Immigration augments the competition, but the migrants are not complicit because they are victims of inequity—of this throwaway economy and of wars," he said

Conflicts in today's world, he said, translate to "a Third World War in segments."

The pope also slammed those who claim to be pushing for peace but who are actually manufacturing or selling weapons.