Top NASA scientists said Monday there was no scientific evidence to support a colleague's claim that fossils of alien microbes born in outer space had been found in meteorites on Earth.

The US space agency formally distanced itself from the paper by NASA scientist Richard Hoover, whose findings were published Friday in the peer-reviewed Journal of Cosmology, which is available free online.

In response, the journal's managing editor Lana Tao lashed out at "truly unprofessional and frankly dishonest conduct of various individual(s) at NASA who have resorted to lies, slander, defamation and ad hominem attacks."

"Hysteria and lies do not constitute scientific doubt. They are calls for medication," she said in an email.

According to the study, Hoover sliced open fragments of several types of carbonaceous chondrite meteorites, which can contain relatively high levels of water and organic materials, and looked inside with a powerful microscope, Field Emission Scanning Electron Microscopy (FESEM).

He found bacteria-like creatures, calling them "indigenous fossils" that originated beyond Earth and were not introduced here after the meteorites landed.

"The implications are that life is everywhere, and that life on Earth may have come from other planets," the study claimed.

Carl Pilcher, director of NASA's Astrobiology Institute, described Hoover as a "NASA employee" who works in a solar physics branch of a lab in the southeastern state of Alabama.

"That is a claim that Mr Hoover has been making for some years," Pilcher told AFP.

"I am not aware of any support from other meteorite researchers for this rather extraordinary claim that this evidence of microbes was present in the meteorite before the meteorite arrived on Earth and and was not the result of contamination after the meteorite arrived on Earth," Pilcher said.

"The simplest explanation is that there are microbes in the meteorites; they are Earth microbes. In other words, they are contamination."

Pilcher said the meteorites that Hoover studied fell to Earth 100 to 200 years ago and have been heavily handled by humans, "so you would expect to find microbes in these meteorites."

Paul Hertz, chief scientist of NASA's Science Mission Directorate in Washington, also issued a statement saying NASA did not support Hoover's findings.

"While we value the free exchange of ideas, data and information as part of scientific and technical inquiry, NASA cannot stand behind or support a scientific claim unless it has been peer-reviewed or thoroughly examined by other qualified experts," he said.

He noted that the paper did not complete the peer-review process after being submitted in 2007 to the International Journal of Astrobiology.

Studies suggesting that microbes from outer space can be found in meteorites date back to the 1960s, but have been widely met with skepticism in the scientific community.

"Unless he has forged those pictures, which I doubt... I cannot imagine that they can be contaminants," said Chandra Wickramasinghe, a Journal of Cosmology editor.

"I know there is skepticism because it is a remarkable discovery if it is validated," he told AFP.

"If it is accepted it means that much of the conventional wisdom on how life started on the Earth has to be revised."

Wickramasinghe said the research was peer-reviewed by four geologists and geochemists from Russia and Europe and it gained "positive responses from three of them."

The journal invited experts to weigh in on Hoover's claim, and both skeptics and supporters began publishing their commentaries on the journal's website Monday.

Michael Engel of the University of Oklahoma wrote: "Given the importance of this finding, it is essential to continue to seek new criteria more robust than visual similarity to clarify the origin(s) of these remarkable structures."

A separate NASA-funded study in December suggested that a previously unknown form of bacterium, found deep in a California lake, could thrive on arsenic, adding a new element to what scientists have long considered the six building blocks of life.

That study drew hefty criticism, particularly after NASA touted the announcement as evidence of extraterrestrial life. Scientists are currently attempting to replicate those findings.