A "piecemeal" World War III may have already begun with the current spate of crimes, massacres and destruction, Pope Francis has warned.

He was speaking during a visit to Italy's largest military cemetery, where he was commemorating the centenary of World War I.

"War is madness," the Pope said at a memorial to 100,000 Italian soldiers at Redipuglia cemetery near Slovenia.

The Argentine Pope has often condemned the idea of war in God's name.

Only last month, Pope Francis said the international community would be justified in using force to stop what he called "unjust aggression" by Islamic State militants, who have killed or displaced thousands of people in Iraq and Syria, including many Christians.

In today's homily, standing at the altar beneath Italy's fascist-era Redipuglia memorial - where 100,000 Italian soldiers killed during WWI are buried, 60,000 of them unnamed, the Pope paid tribute to the victims of all wars.

"Humanity needs to weep, and this is the time to weep," he said.

"Even today, after the second failure of another world war, perhaps one can speak of a third war, one fought piecemeal, with crimes, massacres, destruction," he said.

The pontiff's visit was also infused with personal meaning.

His grandfather fought in - and survived - Italy's offensive against the Austro-Hungarian empire, in north-east Italy in 1917 and 1918.

In recent months the Pope has called for an end to conflicts in Iraq, Syria, Gaza, Ukraine, and parts of Africa.