During two weeks in 1975, a string of the nation's supersensitive nuclear missile launch sites and bomber bases were visited by unidentified, low-flying and elusive objects, according to Defense Department reports.

The sightings, made visually and on radar by air and ground crews and sabotage-alert forces, occurred at installations in Montana, Michigan and Maine, and led to extensive but unsuccessful Air Force attempts to track and detain the objects.

Air Force and Defense Department records variously describe the objects as helicopters, aircraft, unknown entities and brightly lighted, fast-moving vehicles that hovered over nuclear weapons storage areas and evaded all pursuit efforts.

In several instances, after base security had been penetrated, the Air Force sent fighter planes and airborne command planes aloft to carry on the unsuccessful pursuit. The records do not indicate if the fighters fired on the intruders.

The documents also give no indication that the airspace incursions provoked much more than local command concern.

But a Nov. 11, 1975, directive from the office of the secretary of the Air Force instructed public information staffers to avoid linking the scattered sightings unless specifically asked.

An Air Force press officer who deals with UFO inquiries said he could have no comment yesterday on questions about general security and military responses related to the rash of sightings at strategic installations in 1975.

The Defense Department postiion, cited in that memo and reiterated yesterday by a departmental spokesman, is that formal investigation of unidentified flying obects (UFOs) ended in 1969 and that there were no plans for renewed Air Force investigation.

Yet another Air Force intelligence report indicated extensive interest in a 1976 incident over Iran, when two Iranian Air Force F4 Phantom fighter planes were scrambled to encounter a brightly lighted object in the skies near Tehran.

That object was tracked by Iranian ground radar, seen independently by the crew of a commercial airliner and pursued by the F4, which, according to the report, experienced a breakdown of their electronic communications devices when they neared the object.

The report, compiled by American officials, said that the electronic weapons system of one of the planes went dead when its pilot prepared to fire an AIM9 missile at a smaller object that appeared to roar out from the larger vehicle.

The planes' electronic equipment reportedly became operative after they veered away from the smaller object, which had returned to the larger light, the report said. Iranians described the larger object, with colored, fast-flashing lights, as the size of a Boeing 707 jetliner.

The information on the 1975 and 1976 sightings -- records from the Air Force and the North American Air Defense Command (NORAD) -- was turned over to Ground Saucer Watch (GSW), a Phoenix-based organization that monitors UFO reports.

GSW obtained the information through a freedom-of-Information request to the Air Force, one of a number it has made to government agencies involved in UFO investigations.

A similar request to the CIA, made both by GSW and The Washington Post, resulted in the CIA's turning over almost 900 pages of documents related to its monitoring of UFO reports since the 1950s.

The CIA was directed by a U.S. District Court judge here last year to turn over to Ground Saucer Watch UFO data unrelated to national security.

The agency, according to GSW officials and attorneys, apparently has withheld some UFO records, and GSW says it intends to seek further court action in the case.

The Air Force and other federal military and intelligence agencies have maintained consistently that sightings of unidentified flying objects have logical explanations -- that the UFOs are not visitors from another world.

The CIA documents are largely a collection of worldwide intelligence reports, newspaper articles and agency memoranda relating to UFO sightings and theories of extraterrestrial life.

The CIA's position, reiterated yesterday by a spokesman, is that it has had no involvement with UFOs since 1953, when a special study panel concluded that they presented no threat to national security.

While memos from as recently as 1977 are included in the 879 pages, the CIA spokesman said the agency continues to be "a passive recipient" of UFO data, even though none of the material is analyzed.

Todd Zechel, a GSW investigator and director of another organization, Citizens Against UFO Secrecy, said, "We've had to pry loose every item of information we have. I am inclined to believe the government doesn't know any more about UFOs than we do, but if UFOs are what they say -- nothing -- why don't they open their files totally?"

Zechel and William Spaulding, a Phoenix engineer and director of GSW, said that Defense, the Air Force and the National Security Agency (NSA) have refused to turn over certain other information that would shed more light on military encounters with unidentified flying objects.

Zechel, a former NSA employe who now lives in Wisconsin, said that the 1975 incidents around the missile and bomber facilities would not have been revealed had it not been for a "leak" from a Pentagon source.

That tip, he said, led to the information request that produced the reports on the "flap," as a rash of UFO incidents is called, in the last days of October and the first two weeks of November 1975.

The Air Force and NORAD data provided detailed accounts of sightings of unexplained objects from Loring Air Force Base in Maine, Wurtsmith AFB in Michigan and Malmstrom AFB in Montana, all with a two-week period.

As those and other missile-launching sites in the northern tier of states, military personnel reported that the objects hovered over nuclear weapons storage areas, in some cases as low as 10 feet from the ground, and missile silos, before they departed.

The reports referred to the objects in some cases as "helicopters," although no witness made a positive identification. The sounds the objects emitted were described as being similar to helicopter noise.

In one such instance, on Nov. 7, 1975, at Malmstrom AFB, Capt. Thomas W. O'Brien, who had just left duty as a missile launch officer, said an aircraft resembling a helicopter approached the silo area.

He and his deputy heard what they thought was a helicopter rotor over the building where they were resting. The unidentified deputy looked out the window and saw "the silhouette of a large aircraft hovering about 10 to 15 feet above the ground" and about 25 feet from the launch-area fence.

He reportedly saw two red and white lights on the front, a white light on the bottom and another on the rear. Darkness prevented him from seeing markings or personnel on the object. The object left after a minute or so of hovering, the report said.

Military crews at two other nearby launch facilities reported moving lights in the air on the same evening, but said they heard no sounds.

NORAD commanders' activity logs during that period of time reported another sighting at another unidentifie launch facility in which witnesses said they saw the object"issuing a black object from, it, tubular in shape." Standard radar surveillance provided no clues as to the presence of anything other than known craft in the area.

More detail appeared in reports of sightings on Oct. 30 and 31 over Wurtsmith AFB, where an"unidentified helicopter" flew around the base and hovered over weapons-storage bunkers.

Investigators subsequently determined that no military, commercial or private helicopters known to be based in the area could have been around Wurtsmith at those times. The crew of a KC135 tanker plane, already airborne, spotted the object near the base and attempted to give chase, but couldn't keep up with it.

Several sightings occurred at the Maine air base as well, where objects hovered over the weapons area.Radar and visual sightings were made, and another KC135 was sent aloft to oversee pursuit efforts by a helicopter borrowed from the Maine National Guard -- loring had none of its own.

The object eventually disappeared toward the Canadian border, where Canadian air force jets were on alert. There was no indication whether the Canadian planes spotted the object.