Nasa on Thursday officially joined the search for UFOs -- but reflecting the stigma attached to the field, the US space agency wouldn't identify the director of the new programme tasked with tracking mystery flying objects.

The official's appointment is the result of a year-long Nasa fact-finding report into what Nasa calls "unidentified anomalous phenomena (UAP)."

"At Nasa, it's in our DNA to explore – and to ask why things are the way they are," Nasa chief Bill Nelson said.

An independent team of 16 researchers concluded in the report that the search for UAPs "demands a rigorous, evidence-based approach."

Nasa is well positioned to play a prominent role, thanks to its satellite capabilities and other technical assets. But the agency stressed in its report that any findings of possible extraterrestrial origin "must be the hypothesis of last resort – the answer we turn to only after ruling out all other possibilities."

"We want to shift the conversation about UAP from sensationalism to science," Nelson said.

Even if Nasa has long explored the heavens, hunting for the origin, identity and purpose of a growing number of unexplained flying objects over planet Earth is bringing unprecedented challenges.

Military and civilian pilots keep offering a multitude of reports on strange sightings. But decades of movies and sci-fi books about aliens mean the entire topic is mostly laughed off by the public as the territory of cranks.

That atmosphere explained the unusual decision by Nasa to decline to identify the lead UAP official's identity.

"We need to ensure that the scientific process and methods are free," said Daniel Evans, who worked on the year-long Nasa report leading to the announcement. "Some of the threats and the harassment have been beyond the pale quite frankly," Evans said. (AFP)