President Trump announced on Monday that the moon would be the next destination for American astronauts, putting the Oval Office’s imprimatur on what other administration officials have said for months.

“It marks a first step in returning American astronauts to the moon for the first time since 1972, for long-term exploration and use,” Mr. Trump said during a brief ceremony at the White House, where he signed what the White House called Space Policy Directive 1.

“This time, we will not only plant our flag and leave our footprints,” Mr. Trump said. “We will establish a foundation for an eventual mission to Mars, and perhaps someday, to many worlds beyond.”

Attendees included Harrison Schmitt, one of the astronauts on Apollo 17, the last piloted mission to the moon.