A secret 'X-files' style dossier of UFO sightings in Northern Ireland has been made public for the first time.

Details revealed under the Freedom of Information Act show that 11 sightings have been reported in the area in the past decade.

They range from bright lights to dome-shaped objects, according to the Ministry of Defence log books that list times, dates, places and descriptions.

The busiest year for possible 'space invader' activity in the province was 1999, with five incidents recorded, including two in Co Antrim within 48 hours.

Other reports include 18 lights moving across the sky and an object the size of a bowling ball with red lines on it that "moved fast, then vanished" .

Describing one incident, a witness told the MoD they saw "two egg shaped objects, with red, blue and green coloured lights", adding it was dome shaped at the top but was flat at the bottom.

Every year in the UK, more than 100 UFO sightings are reported to the MoD.

Although the Ministry does not have any expertise in respect of UFOs, it is required to investigate every sighting in order to establish whether or not the UK's airspace has been infringed upon.

An MoD spokesman said: "The MoD examines reports solely to establish whether UK airspace may have been compromised by hostile or unauthorised military activity.

"If required, sighting reports are examined with the assistance of the department's air defence experts.

"Unless there is evidence of a potential threat, there is no attempt to identify the nature of each sighting reported."

However, correspondence between the MoD and members of the public reveals that Whitehall officials remain "totally open- minded" about the existence of UFOs.

Giving details of the MoD's policy on UFOs, the Director of Air Staff said in a lengthy response to a Freedom of Information request: "The Ministry of Defence does not have any expertise or role in respect of 'UFO (or) flying saucer' matters or to the question of the existence or otherwise of extraterrestrial life forms, about which it remains totally open-minded.

"To date the MoD knows of no evidence which substantiates the existence of these alleged phenomena.

"The MoD examines any 'UFO' reports it receives solely to establish whether what was seen might have some defence significance; namely, whether there is any evidence that the United Kingdom's airspace might have been compromised by hostile or unauthorised air activity.

"Unless there is evidence of a potential threat to the United Kingdom from an external source, and to date no 'UFO' report has revealed such evidence, we do not attempt to identify the precise nature of each sighting reported to us."

He added: "We believe that rational explanations, such as aircraft lights or natural phenomena, could be found for them if resources were diverted for this purpose, but it is not the function of the MoD to provide this kind of aerial identification service.

"It would be an inappropriate use of defence resources if we were to do so."

Details of the UFO sightings by members of the public were released in response to a series of requests made under the Freedom of Information Act, the MoD confirmed.