A new de-classified US government report on UFO sightings by US troops has revealed hundreds of new cases.

The US National Intelligence office is now aware of 510 reported sightings, an increase over the 144 compiled in the spy agency's first 2021 assessment.

Nearly half of the new sightings were deemed "unremarkable" and attributed to human origins, according to the report.

However, more than 100 of the encounters remain unexplained.

The report says that encounters with UFOs - which the government calls Unidentified Aerial Phenomena (UAPs) - continue "to occur in restricted or sensitive airspace, highlighting possible concerns for safety" and national security.

Of the 366 new reports, 26 were determined to be drones, 163 were balloons and six aerial objects were attributed to clutter.

The report was issued in part to help "destigmatise" experiences with UFOs and improve air safety.

And it says increased reports of encounters are indeed the result of "a concentrated effort to destigmatise the topic of UAP and instead recognise the potential risks that it poses as both a safety of flight hazard and potential adversarial activity", the report states.

It goes on to say that 171 sightings still remain "uncharacterised and unattributed" - meaning, not enough information was collected to effectively identify them.

"Some of these uncharacterised UAP appear to have demonstrated unusual flight characteristics or performance capabilities, and require further analysis," the report says.

None of the reports have been linked to any extra-terrestrial activity.

The reports are being examined by the All-domain Anomaly Resolution Office (AARO), an office in the Pentagon created last year to review UAP incidents.

The AARO will focus on receiving and analysing incidents with unidentified phenomena and work with intelligence agencies to further assess those incidents, according to the new declassified document.

Last month, the Pentagon's Undersecretary of Defense for Intelligence and Security told media: "We have not seen anything that would lead us to believe that any of the objects that we have seen are of alien origin."

"I have not seen anything in those holdings to date that would suggest that there has been an alien visitation, an alien crash, or anything like that," added Ronald Moultrie.

But the effort continues to identify whether the remaining UFOs have earthly origins.

"In the absence of being able to resolve what something is, we assume that it may be hostile," he said. "And so, we have to take that seriously."