U.S. reviews Syria safe zones but warns 'devil's in the details'

U.S. Defense Secretary Jim Mattis speaks at the opening of the 53rd Munich Security Conference in Munich, Germany, February 17, 2017. REUTERS/Michael Dalder

COPENHAGEN (Reuters) - U.S. Defence Secretary Jim Mattis said on Monday the United States would closely examine proposed de-escalation zones aimed at easing Syria's civil war but warned "the devil's in the details" and that much needed to be worked out.

The deal to create "de-escalation" zones in the major areas of conflict in western Syria took effect on Saturday.