Trump Snaps at Reporter During Oval Office Briefing, 'You Better Be Careful'

President Donald Trump laid down the law with a reporter in the Oval Office ... letting his inner New Yorker out by telling the interviewer to watch it.

POTUS was taking questions from reporters when he pointed in the general direction of the scrum, and a journo started asking a question ... and things quickly went off the rails.

Trump went nuclear on the guy -- cutting him off and chastising him in front of his colleagues and a room full of microphones and cameras.

The prez said, "I don't know who the hell you are, but you better be careful." He then told a different member of the media to ask their question.

Clearly, Trump felt the guy was butting in when he wasn't called upon -- though it seems to be an overreaction to an honest mistake. After all, Trump didn't call on anyone by name ... he just pointed toward a tight part of the group.

Of course, part of the reason Trump has to take so many questions from reporters is that the White House just lost another press secretary.

As we've reported ... Katie Zacharia had the job for one work week, then got canned during an interview with The Wall Street Journal -- all while Trump was telling reporters he told her she didn't have to keep the job if she didn't want to.