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.

President Donald Trump accused former president Barack Obama of disclosing "classified information" after Obama said in a recent podcast interview that aliens are "real".

He wrote: "Based on the tremendous interest shown, I will be directing the Secretary of War, and other relevant Departments and Agencies, to begin the process of identifying and releasing Government f...