As the UN secretary-general visited the world's biggest camp for Syrian refugees in late March, he made an impassioned plea: stop Syria's devastating war.

"I want to make a strong appeal to the parties of the conflict and those who have an influence on to understand that we must make peace," Antonio Guterres, a former Portuguese prime minister, told reporters at the camp on the eve of a summit gathering Arab leaders on the shores of the Dead Sea in Jordan.

"This has not only become a tragedy for the Syrian people, but it has become a threat to the stability of the region, a global security threat for the world, as terrorism is benefiting from the crisis in Syria and other crises in the world," he said.