President Donald Trump gives thumbs up before departing Shannon Airport Ireland. Picture: Alex Brandon/AP

Washington - US President Donald Trump has contradicted NASA's stated goal of returning humans to the moon.

"For all of the money we are spending, NASA should NOT be talking about going to the Moon - We did that 50 years ago," Trump tweeted. "They should be focused on the much bigger things we are doing, including Mars (of which the Moon is a part), Defense and Science!" 

NASA announced earlier that it plans to open the International Space Station (ISS) to commercial space travel. In making the announcement NASA officials reiterated the goal of returning humans to the moon by 2024.

Trump posted the tweet from Air Force One en route to Washington following his trip to Europe to mark the 75th anniversary of the D-Day invasion.