US President Donald Trump has accused Barack Obama of revealing classified information when the former president said "aliens are real" in a podcast interview released last week.

"He's not supposed to be doing that," Trump told reporters aboard Air Force One on Thursday, adding: "He made a big mistake."

Asked if he also thinks aliens are real, Trump answered: "Well I don't know if they're real or not."

Obama told podcast host Brian Tyler Cohen that he thinks aliens are real, later saying he thinks its statistically likely for there to be life beyond Earth given the vastness of the universe.

"They're real, but I haven't seen them, and they're not being kept in Area 51. There's no underground facility unless there's this enormous conspiracy and they hid it from the president of the United States," Obama said.