Radiation levels measured by NASA's Mars Curiosity rover show astronauts probably would exceed current US exposure limits during a roundtrip mission to Mars, scientists said on Thursday.

The rover landed on Mars in August to search for habitats that could have supported past microbial life.

Results taken during Curiosity's eight-month cruise to Mars indicate that astronauts would receive a radiation dose of about 660 millisieverts during a 360-day roundtrip flight, the fastest travel possible with today's chemical rockets. That dosage does not include any time spent on the planet's surface. A millisievert is a measurement of radiation exposure.

NASA limits astronauts' increased cancer risk to 3 percent, which translates to a cumulative radiation dose of between about 800 millisieverts and 1,200 millisieverts, depending on a person's age, gender and other factors.

"Even for the shortest of (Mars) missions, we are perilously close to the radiation career and health limits that we've established for our astronauts," NASA's chief medical officer Richard Williams told a National Academy of Sciences' medical committee on Thursday.

An astronaut living for six months on the International Space Station, which flies about 250 miles (400 km) above Earth, receives a dosage of about 100 millisieverts.

An abdominal X-ray scan generates about 10 millisieverts.

At NASA's request, the Institute of Medicine panel is looking into ethics and health standards for long-duration spaceflights.

"We're looking at that 3 percent standard and its applicability for exploration-type missions," added NASA's Edward Semones, spaceflight radiation health officer at the Johnson Space Center in Houston, during a conference call later with reporters.

"The snapshot today is that we would exceed our limit," Semones said.

NASA also is looking into alternative propulsion technologies to speed up the trip to Mars and different types of spacecraft shielding.

Information from Curiosity about how much and what type of radiation astronauts can expect on the Martian surface is due to be released later this year.

The research was published in this week's edition of the journal Science.