American authorities have examined around 800 mysterious reports of unidentified flying objects collected over decades - but only a small fraction are truly unexplained, a panel of researchers says.

Nasa set up the panel last year to explain its work on what it calls unidentified anomalous phenomena (UAP).

UAP are defined as sightings "that cannot be identified as aircraft or known natural phenomena from a scientific perspective".

The panel held its first public meeting on Wednesday.

Here are some of the revelatory moments.

"We have 50 to 100-ish new reports each month," said Sean Kirkpatrick, director of the All-domain Anomaly Resolution Office (AARO), part of the US Defence Department.

But he said the number of those sightings which are "possibly really anomalous" are 2% to 5% of the total database.

At one point during the hearing, a video taken by a naval aircraft over the western US showed a series of dots moving across the night sky. The military plane was unable to intercept the object, which turned out to be a commercial aircraft heading towards a major airport.

Other sightings are more mysterious.

Mr Kirkpatrick also noted that privacy concerns limit the agency's investigations.

"We can point the largest collection apparatus in the entire globe at any point we want," he said.

"A lot of what we have is around the continental United States," he added. "Most people...don't like it when we point our entire collection apparatus at your backyard."

UAP-related data is often difficult to interpret and can be easily skewed.

David Spergel, chair of Nasa's UAP team, mentioned a burst of radio waves picked up by researchers in Australia.

"They had really strange structure. People couldn't figure out what was going on. Then they start to notice a lot of them bunched together around lunch time," he said.

It turned out that the sensitive instruments used by researchers were picking up signals from a microwave used to heat up their lunches.

Scott Kelly, a former astronaut and pilot with decades of experience, told a story about an optical illusion.

He and his co-pilot were flying near Virginia Beach and his colleague "was convinced we flew by a UFO".

"I didn't see it. We turned around, we went to look at it, it turns out it was Bart Simpson - a balloon."

Commercial pilots are very reluctant to report sightings, Mr Spergel said, because of the stigma surrounding flying saucers.

"One of our goals is to remove the stigma," he said, "because there is a need for high quality data to address important questions about UAPs."

And some scientists have faced harassment online for their work in the area.

"Harassment only leads to further stigmatisation of the UAP field, significantly hindering the scientific process and discouraging others to study this important subject matter," said Nasa science chief Nicola Fox.

One of the reasons Wednesday's meeting is so noteworthy is Nasa's change of approach. The space agency spent decades debunking UFO sightings.

At the end of the hearing the panel took questions from the public. One was "What is Nasa hiding?"

Nasa's Dan Evans replied that the agency is committed to transparency. "That is why we are here live on TV today," he said.