NASA downplayed Wednesday talk of a major discovery by its Martian rover after remarks by the mission chief raised hopes it may have unearthed evidence life once existed on the Red Planet.

Excitement is building over soon-to-be-released results from NASA's Curiosity rover, which is three months into a two-year mission to determine if Mars has ever been capable of supporting microbial life.

Its Sample Analysis at Mars (SAM) instruments have been sending back information as it hunts for compounds such as methane, as well as hydrogen, oxygen and nitrogen, that would mean life could once have existed there.

In an interview with US broadcaster National Public Radio, aired Tuesday, lead mission investigator John Grotzinger hinted at something major but said there would be no announcement for several weeks.

"We're getting data from SAM," he said. "This data is gonna be one for the history books. It's looking really good."

A spokesman for NASA's Jet Propulsion Laboratory, which is managing the project, appeared to pour cold water Wednesday on the hopes of space enthusiasts looking forward to an earth-shattering discovery.

"John was delighted about the quality and range of information coming in from SAM during the day a reporter happened to be sitting in John's office last week. He has been similarly delighted by results at other points during the mission so far," spokesman Guy Webster told AFP.

"The scientists want to gain confidence in the findings before taking them outside of the science team. As for history books, the whole mission is for the history books," Webster said.

Scientists do not expect Curiosity to find aliens or living creatures but they hope to use it to analyze soil and rocks for signs the building blocks of life are present and may have supported life in the past.

The $2.5 billion Curiosity rover -- which landed in Gale Crater on the Red Planet on August 6 -- also aims to study the Martian environment to prepare for a possible human mission there in the coming years.

US President Barack Obama has vowed to send humans to the planet by 2030.