Mattis: US reviewing Syria safe zones but has many questions

Defense Secretary Jim Mattis says the U.S. will take a close look at Russia's plan to create several safe zones in Syria.

Russia, Turkey and Iran have agreed to enforce a cease-fire between Syrian government and opposition forces in four locations. Russia says map details won't be published until early June.

Traveling with reporters to Copenhagen, Mattis tells reporters that the "devil is always in the details."

Mattis says of the proposed safe zones: "Who is going to be ensuring they're safe? Who is signing up for it? Who is specifically to be kept out of them? All these details are to be worked out and we're engaged."