The strikes were on external actors who were actively "plotting attacks in Europe," Secretary of Defense Ash Carter told reporters.
The mission was approved several days ago by President Barack Obama and is expected to be the last short-notice military operation ordered by him, according to sources.
"The fighters training in these camps posed a security risk to Libya, to its neighbors, to our allies in Africa and Europe, and to the United States," Pentagon spokesman Peter Cook told reporters.
"While we are still evaluating the results of the strikes, the initial assessment indicates they were successful."
A US defense official said the militants were seen immediately beforehand carrying weapons, wearing tactical vests, holding mortars and standing in formation.
The Department of Defense showed reporters a rare video of surveillance footage of the ISIS fighters as they loaded what appeared to be shells and rocket-propelled grenades into pick-up trucks. A second video showing the camps being bombed was also shown.
The number killed was not released by the Pentagon, but the first estimate put the figure at over 80.
There were no women or children in the area, the official said.
Cook described the camps as being in a "very rural area."