WASHINGTON - US President Donald Trump on Monday signed his administration's first space policy directive, formally directing the US space agency NASA to send astronauts back to the moon and eventually Mars.

"This time, we will not only plant our flag and leave our footprint," Trump said at a White House ceremony.

"We will establish a foundation for an eventual mission to Mars, and perhaps, someday, to many worlds beyond," he said.

Trump offered no specific timeline during his four-minute speech.

The Space Policy Directive 1, as it's called, was based on recommendations of the National Space Council, which Trump directed to reestablish in June to advise and help implement his space policy with exploration as a national priority.

The council was first created in 1989 during the administration of President George H.W. Bush but later disbanded in 1993 under President Bill Clinton.

It marked a return to the vision of President George W. Bush, whose Constellation program aimed to return astronauts to the moon by 2020 and then go on to Mars.

However, Constellation was cancelled in 2010 by then-President Barack Obama, on the grounds that Americans "have been there before" and that "there's a lot more of space to explore."