Defense Secretary James Mattis said Friday the U.S. is confident that Syria still has chemical weapons, and warned Syria not to use them again on its own people.

"There can be no doubt in the international community's mind that Syria has retained chemical weapons in violation of its agreement, and it's statement that it had removed them all," Mattis said in Israel in a press conference with Israel's defense minister.

"There's no longer any doubt," he said. "The amount of it I don't want to get into right now. We don't reveal some of that detail because we don't want to reveal how we're finding out."

He said Syria's decision to keep those weapons is a violation of the United Nations Security Council, and said the issue would have to be "taken up diplomatically."

But he also indicated that more U.S. strikes could follow if Syrian President Bashar Assad tries to use those weapons again.

"They'd be ill-advised to try to use any of that again," he said. "We've made that very clear with our strikes."