Pope Francis called Sunday on warring parties in the Middle East, Iraq and Ukraine to put an end to violence which is wounding and killing countless children.

"Stop, please stop! I beg you with all my heart," the 77-year-old Argentine pontiff pleaded before pilgrims packed in St Peter's Square for the weekly Angelus prayer.

"I think of the children, who are robbed of the hope of a dignified life, of a future. Dead children, wounded children, mutilated children, orphans, children who, for toys, have the debris of war. Children who do not know how to smile," he said.

Marking the 100th anniversary of the outbreak of World War I, Francis called on people "not to repeat the mistakes of the past," saying: "Let us remember that everything is lost in war, nothing is lost in peace."