US President Barack Obama on Monday ribbed scientists behind NASA's roving robot Curiosity, instructing them to let him know right away if they found life on Mars.

"If in fact, you do make contact with Martians, please let me know right away," Obama joked, as he called the scientists at NASA's Jet Propulsion Laboratory in California from Air Force One.

"I've got a lot of other things on my plate, but I suspect that that will go to the top of the list. Even if they're just microbes, it will be pretty exciting."

Curiosity touched down on Mars last week, after a high-risk landing, and will hunt for soil-based signatures of life on Earth's nearest neighbor and send back data to prepare for a future human mission.

It will also haul a Mars Science Laboratory as far as halfway up Mount Sharp, a towering three-mile (five-kilometer) Martian mountain with sediment layers that may be up to a billion years old.

Obama said that thanks to the dedication of NASA scientists, Curiosity "stuck her landing and captured the attention and imagination of millions of people not just across our country, but people all around the world.

"You've made NASA proud. You guys are examples of American know-how and ingenuity, and it's really an amazing accomplishment," he said, and offered a political vow to maintain critical investment in science and technology.

Obama's call was another example of the advantages of political incumbency: he got to talk to the scientists in his official role from his presidential plane while his Republican foe Mitt Romney only got to talk about them.

"We just landed on Mars and took a good look at what's going on there," Romney said in Florida on Monday.

The Republican also lauded America's 1960s missions to the moon and suggested that China, which plans to land a probe on the moon next year for the first time, "take a good look at our flag that was flown there 43 years ago."