President Obama on Thursday reiterated his assertion that Donald Trump is too temperamental to serve as commander in chief.

"I would ask all of you to just make your own judgment," Obama told reporters gathered at the Pentagon to hear the latest update on the coalition's fight against the Islamic State.

"I've made this point already, multiple times," he said. "Just listen to what Mr. Trump has to say and make your own judgment with respect to how confident you feel about his ability to manage things like our nuclear triad."

When pressed further on whether Trump can be trusted with the nuclear codes, Obama said the question had already been asked and answered.

"As I recall, I just answered a question about this a couple of days ago, and I thought I made myself pretty clear," Obama said before heading back to Washington to celebrate his 55th birthday. "And I don't want to just keep repeating it, or a variation on it. I obviously have a very strong opinion about the two candidates who are running. One is very positive and one is, not so much."

If reporters keep asking Obama whether he believes Trump is fit for office, they will get the same answer, he said.

"I think you'll hear pretty much variations on the same theme. What I can say is: This is serious business. And the person who is in the Oval Office and who are secretary of defense and are Joint Chiefs of Staff and our outstanding men and women in uniform report to, they are counting on somebody who has the temperament and good judgment to be able to make decisions to keep America safe. And that should be very much on the minds of voters when they go into the voting booth in November," Obama said.

During a news conference last week, Obama said that many of Trump's controversial proposals and personal attacks on critics prove that the GOP nominee is unfit for office.

"But there has to come a point at which you say somebody who makes those kinds of statements doesn't have the judgment, the temperament, the understanding, to occupy the most powerful position in the world," Obama said.