A Russian colleague (Mikhail Gershtein) has kindly provided me with some US material that I have not previously seen: UFO articles from the US Department of the Navy's Office of Naval Research's "Research Reviews" and the US "Air Force and Space Digest".
The US "Air Force and Space Digest : The Magazine of Aerospace Power" was published by the Air Force Association. The November 1959 issue of that publication included an article by Donald H Robey, "a young physicist associated with the Convair-Astronautics Division of General Dynamics". The article suggested that flying saucers "may be the remnants of space- born icebergs which many scientists now believe are the stuff of those eerie visitors from the void—comets...".
"Research Reviews" was published by the US Department of the Navy's Office of Naval Research. An issue from February 1957 includes a UFO article by Urner Liddel, the head of the Nuclear Physics Branch of the ONR. He wrote that reports of UFOs occasionally "served a rather useful purpose, for they have helped us to locate valuable equipment flown by the large SKYHOOK balloons".
These articles should be read in conjunction with the articles I recently uploaded from "The Airman", the "official magazine of the US Air Force".
I have added these articles to the folder I recently created for UFO articles from mainstream (i.e. non-UFO) publications in the USA, which is separate to the older (and much larger) folder in the online archive for UFO periodicals from the USA.