The 16-member body, formed last year among leading experts from scientific fields ranging from physics to astrobiology, held a four-hour session streamed live on a NASA webcast to deliberate their preliminary findings ahead of issuing a report expected later this summer.

The panel's chairman, astrophysicist David Spergel, said his team's role was "not to resolve the nature of these events", but rather to give NASA a "roadmap" to guide future analysis.

NASA officials said several panelists had been subjected to unspecified "online abuse" and harassment since beginning their work in June last year.

"It is really disheartening to hear of the harassment that our panelists have faced online because they're studying this topic," NASA's science chief, Nicola Fox, said in her opening remarks. "Harassment only leads to further stigmatization."

The greatest challenge panel members cited, however, was a dearth of scientifically reliable methods for documenting UFOs, typically sightings of what appear as objects moving in ways that defy the bounds of known technologies and laws of nature.

The underlying problem, they said, was the phenomena in question were generally being detected and recorded with cameras, sensors and other equipment not designed or calibrated to accurately observe and measure such peculiarities.

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