As close encounters go, the claim by a woman that an alien attempted to seduce her on a country road sounds far-fetched, but files released today reveal that it was considered serious enough for the Ministry of Defence to investigate.

The Norfolk woman who claimed she was approached by a man who said he came from another planet similar to Earth was questioned by the MoD intelligence branch DI55, whose brief was to investigate credible UFO reports.

She told them that, during their 10-minute chat, the man said his race was responsible for creating crop circles and explained the importance of contact between humans and his own people.

The woman, whose identity is not revealed in the government's "X-Files", was described by officials as "agitated" following the incident in which she "heard a loud buzzing noise behind her, then turned to witness a large, glowing spherical object rise steadily until it disappeared".

The claims were examined by DI55 (whose very existence was denied by the government until recently) and described by an intelligence official as "one of our most unusual UFO reports".

Other cases include the description of a black inverted boomerang-shaped UFO by two experienced air traffic controllers at Heathrow. The sighting, from the airport's control tower on the morning of 17 December 1992, came a week after numerous witnesses in Louth, Lincolnshire, reported seeing three lights attached to a large, triangular craft.

Today's release of the MoD documents also sheds light on one of Britain's most infamous UFO episodes, namely the death of an American air force pilot, Captain William Schaffner, whom conspiracy theorists believe was killed during a high-speed duel with aliens above the North Sea.

On the evening of 8 September 1970, Schaffner's RAF Lightning crashed into the sea during a low-level exercise following take-off from RAF Binbrook, Lincolnshire. Schaffner's body was never found, which became significant after claims that Schaffner's plane was scrambled to intercept UFOs.

Although the 28-year-old's death continues to be attributed by some internet sites to a secret war between aliens and earthlings, the MoD files contain a previously unreleased summary of the original RAF Board of Inquiry report into the crash, which makes no mention of UFOs. It concluded that the pilot's death was a tragic accident.

Dr David Clarke, a lecturer in journalism at Sheffield Hallam University and author of Flying Saucerers: A Social History of UFOlogy, said: "From suspected US Air Force spy planes to Russian rockets burning up in the atmosphere, these new files show the many and varied explanations for UFO reports submitted to the MoD. Making the material available allows us all to make an informed decision on the mystery of UFOs."

One of the themes to emerge from these papers is the Aurora spy plane saga, with the files containing a little-known set of colour photographs, apparently taken in the Scottish Highlands, which appear to show a large diamond-shaped UFO shadowed by military jets.

During the 1980s, rumours abounded about a spy plane codenamed Aurora, said to be capable of hypersonic speed. Although the US authorities denied its existence, alleged sightings frequently made headlines in UFO magazines. The DI55 papers confirm that officials also believed it was possible that someone was flying an advanced aircraft within UK air space.

The most intriguing incident involving such a craft occurred at 9pm on 4 August 1990, at Calvine, a remote hamlet near Pitlochry in Scotland. According to the brief details released by the MoD, witnesses saw a diamond-shaped UFO hovering for about 10 minutes before it disappeared upwards at high speed. During the incident, Harrier jump jets were seen making a number of low-level passes. Colour photographs reveal both the UFO and at least one of the jets.

Former MoD official Nick Pope described the photographs as "one of the most intriguing [UFO] cases in the MoD's files".

However, Clarke said the papers failed to address long-standing questions over the incident. He said: "Many questions remain. Who was the photographer and how can we be sure his story was genuine? Why did the Daily Record decide not to publish the photographs in 1990? If they really were taken on the date stated, then why was the MoD unable to trace the origin of the Harriers clearly shown in the print?

"All we have is the usual rather bland statement that the MoD decided the incident was unexplained, but of no defence significance: case closed."