U.S. Defense Secretary Jim Mattis says the Pentagon will not disclose how much damage occurred when the military used its most powerful non-nuclear bomb to strike an Islamic State stronghold in Afghanistan.

The retired four-star Marine general says the Pentagon learned from its Vietnam war experience that it doesn't pay to judge the success of battlefield action in terms of the number of enemy forces killed. It was a reference to the so-called body count that was publicly exaggerated in Vietnam.

Instead, he says the April 13 use of the so-called "mother of all bombs" shows the U.S. doing "what was necessary to break ISIS," whether in Afghanistan, Iraq, Syria or elsewhere.

Mattis was asked about the bombing by journalists traveling with him Thursday to Tel Aviv.