U.S. Secretary of State John Kerry says the stability of the world is directly linked to climate change and its impact on food security for billions of people.

In a speech to the Milan Expo on Saturday, Kerry argued that unrest tied to climate change-induced agricultural failure poses an international threat. He said drought-spawned mass urban migration in Syria exacerbated political tensions ahead of the start of that country's civil war.

The Milan Expo is focused on food security, and Kerry urged attendants to act quickly against climate change. He said such action is not just a matter of food security but of global security.

Kerry is in Italy on the first leg of a three-nation tour of Europe that will also take him to France and Spain.