Barack Obama thinks Stephen Colbert would make a "significantly better" US president than some previous commanders-in-chief.

The former president - who had two terms in office between 2009 and 2017 - appeared on The Late Show ahead of its final episode later this month and he and the host debated the presenter's chances of being voted into office for his next job.

During their pre-taped interview, Colbert said: "I'm looking for a new gig soon, and a lot of people tell me I should run for president."

Obama replied: "Well, you certainly have the look. You have the hair."

Colbert added: "For the record, I think it's a stupid idea. How dumb do you think it is for people to say I should run for president?"

Obama replied: "The bar has changed. Put it this way: I think you could perform significantly better than some folks that we've seen. I have great confidence in that."

Although the 64-year-old former US leader didn't directly speak about the country's current president, Donald Trump, he spoke of the powers he doesn't think the person in office should have.

He said: "We're going to have to do some work to return to this norm, and we probably now have to codify it. The White House shouldn't be able to direct the attorney general to go around prosecuting whoever the president wants prosecuted.

"Two of the core principles of a democracy. We can survive a lot, bad policy, funky elections, there's a bunch of stuff that we can overcome. We can't overcome the politicisation of the criminal justice system, the awesome power of the state. You can't have a situation in which, whoever's in charge of the government, starts using that to go after their political enemies, or reward their friends, right?"

"Another power, maybe, I'd say would be, although this is in the Constitution, it's a little hard to change, but, maybe don't pardon people who've given you a bunch of campaign contributions.

"So, restoring some sense of the Justice Department being independent...

"The second thing is our military. Don't politicise our military."

Earlier this year, Obama caused a stir when he said aliens were "real", and while he admitted he hopes there is life on other planets, he insisted it would be impossible to keep any evidence of UFOs on Earth a secret.

He said: "For those of you who still think that we've gotten little green men underground somewhere, one of the things you learn as president is the government is terrible at keeping secrets.

""This idea of conspiracy theories--if there were aliens or alien spaceships or anything under the control of the United States government… I promise you some guy guarding the installation would have taken a selfie with one of the aliens and sent it to his girlfriend to impress her. There would be leaks."

And Obama thinks he's be a good representative to meet with extraterrestrials.

He said: "First contact, I think I would be a good emissary for the planet."