Britain has posted 19 previously-secret files on a government Web site detailing - in some cases - what are thought to be close encounters between aircraft and unidentified flying objects.

In one instance 51 years ago, two U.S. fighter planes were scrambled over a Royal Air Force base east of London, and one of the planes was seconds away from firing 24 missiles at the UFO.

Retired U.S. pilot Milton Torres, now living in Miami in the southern U.S. state of Florida, said the object moved at speeds approaching 12,000 kilometers an hour and then simply disappeared moments before he was to open fire. He said he was later visited by a government official who instructed him never to talk about the incident.

In a separate file, an Alitalia pilot, Achille Zaghetti, sighted a flying object streaking across the skies in front of his jetliner while flying in 1991 from Milan to London.

An investigation failed to explain the sighting.

The files, posted on Britain's National Archives Web site, offer no official explanation for the two incidents. But they link other UFO sightings to weather balloons, clouds and normal aircraft.

The posting marks the second time this year that Britain has released UFO files. The Associated Press quotes one UFO expert as saying about 160 UFO files will be released over the next several years.

Experts say the vast majority of UFO sightings come from members of the public who see things such as aircraft lights, satellites and meteors. Those sightings are often interpreted as unexplained phenomena, despite logical explanations for what has been observed.

Some information for this report was provided by AP and Reuters.