US President Donald Trump has ordered all government agencies, including his Department of Defense, to "conduct a thorough investigation" into the release of information they have about aliens.

Trump announced the order on his social media page after accusing former President Barack Obama of leaking classified information in the matter.

Obama said in a podcast interview last week that "aliens exist."

Obama told reporters aboard Air Force One that he "should not have leaked the secret." The president also criticized the president for making a "big mistake."

When asked if aliens exist, Trump said, "I don't know if they exist or not."

Former President Barack Obama said in a podcast broadcast last Saturday that aliens exist.

"They exist; they are real. But I haven't seen them," he said, adding, "Unless there is a secret underground space that is being kept from the president of the United States, as the conspiracy theory suggests, there is no secret place."

After the former president's comments sparked a debate, Obama explained that he believes that the universe is vast enough to support life beyond Earth.