Christian de Duve, an expert on life in the universe, said the evidence presented by Nasa scientists at a news conference on Wednesday was far from conclusive.
"Just because organic substances were found on a meteorite that is supposed to have come from Mars does not mean they were formed on Mars by living organisms," said Mr de Duve.
"The scientists were very cautious in saying there could be other interpretations. But they did participate in the show, which was obviously stage-managed by Nasa ... Would this have happened if this was not an election year in the US?"
A human mission to Mars is a long-term goal of the US space programme, but Nasa says any long-duration mission would have to follow the building of an $27bn international space station - not expected to be completed until 2002.
Scientists say, however, that humans will have to travel to Mars to confirm the theories about extraterrestrial life. "I feel in my stomach, my guts, that we have a great potential to do it, and in the end we will probably send people there, "said Nasa administrator Daniel Goldin. The US plans to send 10 unmanned probes to Mars over the next 10 years, he said.
In the UK, science and technology minister, Ian Taylor, said Britain could contribute "a wealth of expertise" in space research and technology. Equipment being built in Oxford will be part of Nasa's 1998 Mars Survey orbiter, which will investigate the layered structure of Mars' atmosphere, such as its temperature, water vapour, and dust content.
Some of the exploration spacecraft will have to include robotic-rovers that can examine the surface of the planet. Underground samples appear increasingly important because of the requirement of water for life on Mars, according to David McKay, a Nasa scientist who has studied Martian meteorites.
"At some point in Mars' history things went bad ... the atmosphere mostly disappeared and the surface water too," he said. "The question is, what happened to this early life? One view is that that early life went underground and may still be there ... the only way we'll know is to go there."
While parts of the space station have been built, none has been put into orbit, and the programme has been a favourite target of budget-cutters in Congress. Space community experts estimated in 1989 that the cost of a human mission to Mars could be as much as $500m.Reuse content