BBC Washington correspondent Stephen Sackur: scientists want the subject to be taken seriously

But the scientists did not go as far as saying there was proof that aliens exist.

Instead the international panel stressed it had found no evidence of extraterrestrial intelligence.

Despite their reservations, the scientists concluded some sightings are accompanied by compelling physical evidence that warrants further study.

"If there is an interest in trying to get serious answers to the UFO problem, it would be sensible for scientists to focus on the physical evidence as opposed to witness testimony," said the panel's director, Peter Sturrock of Stanford University.

The report says the scientific community might learn something worthwhile if it overcame a fear of ridicule associated with the topic and got some funding for targeted research.

"It may be valuable to carefully evaluate UFO reports to extract information about unusual phenomena currently unknown to science," said the report.

The report was put together by a group of astronomers, physicists and experts in other scientific disciplines.

Sightings 'not easily explained'

It is the first independent review of UFO phenomena since 1968, when the US Air Force commissioned the University of Colorado to conduct a scientific study of UFOs.

The panel examined evidence including photographs of purported UFOs, radar data and reports of soil damage near supposed UFO landing sites.

Some of the reports could have been explained by rare natural phenomena such as electrical activity above thunderstorms. Others were due to secret military activities.

But it concluded that some of the phenomena related to UFOs could not be easily explained.

Most current UFO investigations lack the level of rigor required by the scientific community, despite the initiative and dedication of the investigators involved.

But the panel said that new data, scientifically acquired and analyzed, could yield useful information and advance our understanding of UFO sightings.

The full report is published in the Journal of Scientific Exploration.