President Donald Trump says he'll destroy the deadly street gang MS-13 as part of his crackdown on undocumented immigrants.

The president made those remarks during a speech before law enforcement officers on Long Island.

Authorities say MS-13 is responsible for at least 17 murders in Suffolk County since January of last year.

Trump thinks the gang is filled with people who are in the U.S. illegally, and that police know who they are.

He promises to back officers 100 percent.

"I said, 'Please don't be too nice,'" Trump said. "Like, when you guys put somebody in the car and you're protecting their head, you know, the way you put their hand ove, like, don't hit their head, and they've just killed somebody. Don't hit their head. I said, 'You can take the hand away, OK?'"

Trump also took a veiled shot at Mayor Bill de Blasio, calling him pathetic for not supporting police.

The mayor took to Twitter to respond to Trump's speech.