"NASA has made a startling discovery that points to the possibility that a primitive form of microscopic life may have existed on Mars more than three billion years ago," NASA administrator Daniel Goldin said.

NASA said in a statement that scientists from the space agency and Stanford University have discovered "strong circumstantial evidence of possible early Martian life, including microfossil remains found in a Martian meteorite."

"I want everybody to know that we are not talking about 'little green men'," Goldin said. "These are extremely small, single-cell structures that somewhat resemble bacteria on Earth. There is no evidence or suggestion that any higher life form ever existed on Mars."

A spokeswoman at the journal Science confirmed that a Mars paper had been received, but she said it would not be published until next week.

Meanwhile, word of the discovery flashed through the astronomy community.

One scientist familiar with the study, commenting only on condition of anonymity, said, "The finding was unequivocal. This could be one of the biggest discoveries ever."

Another scientist, also speaking only anonymously, said he was skeptical but if the report could be confirmed it would be a major discovery.

Past or present existence of life on Mars has been considered a possibility ever since studies by spacecraft landers showed that water was once present on the planet surface.

The new findings center on a meteorite called Allan Hills 84001, the oldest of 12 pieces of rock that earlier studies confirmed as originating from Mars.

It is thought the rocks were jolted away from Mars by some massive collision in ancient times and then drifted in space until they fell to Earth.

Allan Hills 84001 was found to have crystallized about 4.5 billion years ago, during the formation of the solar system.

It is thought to have been knocked from the Martian surface about 15 million years ago and then to have smashed into the icy surface of Antarctica about 13,000 years ago.

The meteorite was discovered in 1984 and later identified as originating from Mars.

Scientists from NASA's Johnson Space Center, from Stanford University, the University of Georgia and from McGill University in Montreal, recently studied thin slices of the rock and found organic molecules called polycyclic aromatic hydrocarbons, or PAHs.

PAHs can be formed only in two ways - by biological action, such as by microorganisms, or in the process that forms planets.