Last year, 'declassified' videos of numerous UFO encounters and reports of a top secret Pentagon program to investigate them made headlines around the world.

Now, fresh details about the bizarre backstory to one of these unexplained incidents have emerged, courtesy of a recently leaked confidential military report that describes "several occasions" where the US Navy detected Anomalous Aerial Vehicles (AAVs) in late 2004.

Sourced by a CBS affiliate investigative team based in Las Vegas, the report exposes numerous mysterious sightings of AAVs between 10–16 November 2004, which is referred to as the USS Nimitz UFO incident.

But as the document makes clear, the incident wasn't just a one-off, but a week-long series of strange run-ins with AAVs, which "would descend 'very rapidly' from approximately 60,000 feet down to approximately 50 feet in a matter of seconds".

Once these AAVs dove, "[t]hey would then hover or stay stationary on the radar for a short time and depart at high velocities and turn rates".

In most circumstances, advanced sensors on board the USS Princeton would track the AAVs during these manoeuvres, until they went out of range.

But in one incident – now immortalised in creepy video footage you can freely watch online – two F/A–18s were vectored to intercept one of the AAVs, with visual contact confirming "an elongated egg or a 'Tic Tac' shape with a discernible midline horizontal axis".

According to the pilots, the craft – if that's what it was – was "solid white, smooth, with no edges… uniformly coloured with no nacelles, pylons, or wings" and estimated to be approximately 46 feet in length.

Per the military assessments made in the document – which at least one observer claims may be a document prepared for the Pentagon, not by it – the AAV was not a known aircraft or vehicle in the inventory "of the United States or any foreign nation … with no visible control surfaces and no visible means to generate lift".

But perhaps the most astonishing new details from the report concern is an account of water disturbance on the calm ocean surface, observed in relation to an AAV incident, estimated to be between 50 to 100 metres in diameter and in a round shape.

"It was the only area and type of whitewater activity that could be seen and reminded [the pilot] of images of something rapidly submerging from the surface like a submarine or ship sinking," the report reads.

"It is possible that the disturbance was being caused by an AAV but that the AAV was 'cloaked' or invisible to the human eye."

Freaky stuff. For now, it's hard to definitively ascertain the report's authenticity, but commentators online are saying the new details demonstrate that "something totally strange did indeed occur."

As for an official explanation of what the report alleges, there is none, with one Pentagon source telling government secrets blog The Black Vault: "We are not able to confirm the authenticity of the document".