We are all complicit in the suffering of refugees if we stand by and remain silent, Pope Francis has said, in some of his strongest words on the crisis yet.

In a statement released ahead of the 2016 World Day for Migrants and Refugees, which will be celebrated by most countries on January 17, the pontiff challenged the international community to respond with compassion to those fleeing war and persecution. So far this year, half a million refugees have crossed the Mediterranean into Europe, many of them fleeing Islamic State in Iraq and Syria.

Alluding to Jesus' call to serve the poor in Matthew 25, the pope said it is vital that we treat refugees with the dignity they deserve. "At the heart of the gospel of mercy, the encounter and acceptance by others are intertwined with the encounter and acceptance of God himself," Francis said. "Welcoming others means welcoming God in person!"

Refugees are merely seeking "a better, more prosperous life" for their families; a desire that we all share, the pope added. They are "our brothers and sisters in search of a better life, far away from poverty, hunger, exploitation and the unjust distribution of the planet's resources, which are meant to be equitably shared by all."

Francis blamed "unacceptable humanitarian crises" across the globe for forcing innocent people to flee their homes, but said we must all take responsibility for what happens to them.

"Indifference and silence lead to complicity whenever we stand by as people are dying of suffocation, starvation, violence and shipwreck," he said. "These are always tragedies, even when a single human life is lost."

Those countries that welcome refugees should seek to help them integrate into society, and "prevent the danger of discrimination, racism, extreme nationalism or xenophobia," the pope added.

"Biblical revelation urges us to welcome the stranger; it tells us that in so doing, we open our doors to God, and that in the faces of others we see the face of Christ himself."

Pope Francis has long been outspoken in his response to the refugee crisis, and has previously called for church buildings to be used to help those in need of shelter.

The Vatican has already taken in a Syrian family of four, and has pledged to take another.