Federal agents documented a mystery “black triangle” flying at speed near a sensitive US site earlier this year, new Pentagon UFO files reveal.

The claimed sighting of the “matte black triangular shape of extremely low aspect ratio” took place at an undisclosed location near a “national security site” in the western United States in June, according to newly-released documents.

A FBI observation report said that two federal agents were stationed at an observation post several miles to the west of the location when they were alerted to the supposed sighting.

They said that they detected a “thermally elevated aerial object” and ran to get a better vantage point.

A statement said: “We initiated a visual survey of the western horizon and almost immediately observed a substantial, non-reflective, dark black triangular object most resembling a delta wing drone.

“We estimated the object’s wingspan at approximately nine to fifteen feet, its depth at six to nine feet, and its thickness at one to two feet.”

The agent noted that the object was “hotter than ambient temperature” but had “no visible exhaust gases”.

Also released was a Department of War memorandum, which recorded the sighting from the federal agents and incorporated a corroborating account from a third official positioned at a separate observation post, who initially made the observation using long range thermal equipment.

The memorandum said that the “unidentified craft” made a vertical ascent for approximately 10 seconds, climbing to an altitude of an estimated 200 to 300ft above the local ridge line.

After “instantaneously” arresting its vertical motion, it “entered a stable hover for an estimated twenty seconds”, the report added, before it “reportedly transitioned from a zero-velocity state directly into high-speed horizontal flight.”

This flight saw it execute a “fluid banking manoeuvre and terrain-hugging descent into a canyon drainage in approximately two to three seconds”.

The observers described its speed as “as fast as a fighter jet” but it generated “no detectable acoustic signature” during the sighting.

The FBI document also included a number of pictures which appear to be an object representing the reported sighting superimposed over the landscape.

Many details in the documents are redacted.

The documents released on Friday by the Pentagon were the latest tranche since Donald Trump directed his administration to give the public full disclosure around what it knows about mysterious objects in the sky.