“It’s all in process right now,” said Mattis, who was offering some of the first extensive public U.S. comments on the agreement reached Friday by Russia, Turkey and Iran. “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.”

According to activists in Syria, there already have been skirmishes on the edge of a large cease-fire zone in the northwest portion of the country. Russia, Turkey and Iran agreed to enforce a cease-fire between Syrian government and opposition forces in four locations, but Russia says map details won’t be published until early June.

The U.S. is not part of the agreement, and the Syrian government and the opposition haven’t signed on to the deal. The plan, however, does not cover areas controlled by Islamic State group militants and U.S.-backed Kurdish groups.

That leaves the U.S. and its allies free to continue the campaign to retake IS-held territory. It doesn’t, however, prevent frictions between Turkish troops and their Syrian allies from clashing or going after the U.S.-backed Syrian Kurds.