US President Donald Trump met here Friday with astronauts who were aboard the Apollo 11 spacecraft, as the country celebrates the 50th anniversary of the historic moon landing.

US astronauts Buzz Aldrin, Mike Collins and the family of Neil Armstrong, the first person to have set foot on the moon, were invited by Trump to the Oval Office to celebrate the Apollo 11 moon landing on July 20, 1969.

The US president also took the opportunity to press NASA Administrator Jim Bridenstine to expedite the mission to reach Mars.

"What about the concept of Mars direct?" Trump asked, referring to an approach of sending a spacecraft to Mars directly from the Earth.

"The challenge is, if we go direct to Mars, there's going to be a lot of things that we haven't yet proven out," Bridenstine answered, adding that setting up a base on the moon and a moon-orbiting space station would be a more foolproof approach.

The United States is holding a number of events to commemorate the historic moon landing.

In Washington D.C., the National Gallery of Art is displaying photographs revealing the lunar landscape as early as the 1850s. Armstrong's spacesuit is back on display at the National Air and Space Museum and as a three-dimensional model online.

Besides, the life-size animation of the Saturn V rocket, which propelled the Apollo 11 astronauts to the moon, looms above the National Mall every night, projected onto the Washington Monument.