Several large, quiet, triangular wing-shaped aircraft of unknown origin have been spotted in the air near Air Force bases in California and elsewhere in the West, reports Aviation Week and Space Technology.

A different aircraft whose engines produce a loud roar that resembles heavy rocket engines also has been spotted, the magazine said.The Air Force declined to confirm or deny that the sightings were of new warplanes.

The publication, the current week's edition that became available Monday, says the triangular aircraft are possibly prototypes of the Air Force B-2 or Navy A-12 aircraft.

The A-12 is a new fighter under development, which has not been displayed. The B-2 is the "Stealth" bomber, which has been displayed, and its shape is well-known.

Aviation Week published what appeared to be pictures of the A-12 without explaining whether they were actual aircraft in flight, artists' conceptions, models or something else.

The magazine said the triangular craft did not resemble the production B-2 with its distinctive sawtooth wing trailing edge.

It quoted "well-qualified observers" as reporting that the aircraft are "fairly flat" and triangular shaped with "a rounded nose, rounded wingtips and probably no vertical tail surfaces."

The observers were not otherwise identified.

One observer in Nevada was quoted as saying the shape of the aircraft was "like a manta ray."

The magazine said there have been at least 11 sightings of the triangular aircraft near Edwards Air Force Base in California and one near Fresno, Calif. It said there have been other sightings of similar aircraft in central Nevada.