Former US President Barack Obama has clarified that the chances Earth has been visited by aliens is "low" after comments he made about extra-terrestrial species caught attention online.

"They're real but I haven't seen them," he told American podcast host Brian Tyler Cohen during an interview published on Saturday.

"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," he added.

Obama has since said his comments were made in line with "the spirit of the speed round" of questions, adding that statistically the chances are that life is out there but he saw "no evidence" while president.

Obama's initial comments were made during a quickfire "lightning" round of questions.

"Are aliens real?" Cohen had asked as his first question of the round for the former president.

Obama's answer was subsequently run in different media outlets and attracted attention online - prompting him to publish a clarifying statement on his official Instagram account on Sunday.

"I was trying to stick with the spirit of the speed round, but since it's gotten attention let me clarify," he wrote in a caption that accompanied the video clip from the podcast.

"Statistically, the universe is so vast that the odds are good there's life out there. But the distances between solar systems are so great that the chances we've been visited by aliens is low, and I saw no evidence during my presidency that extraterrestrials have made contact with us. Really!"

Obama, who served as president of the United States between 2009 and 2017, has previously made comments about aliens and UFOs - telling late night TV host James Corden in 2021 that on coming into office he asked whether there was a lab "where we're keeping the alien specimens and space ship".

"They did a little bit of research and the answer was no," he said - but also pointed to records and footage of objects in the sky that had not been explained at the time.