Kyodo News said it discovered documents showing that U.S. intelligence officials had information that during the Korean War, the three international superpowers planned to attack Japan from the air and the ocean, while also invading Taiwan.
The documents found their way into the National Archives after the U.S. officials shared the information with Britain in 1951.
U.S. officials said the report could have originated from the Japanese Communist Party, in an attempt to boost morale by indicating the Soviet Union's involvement.
History experts dismissed the documents' accuracy, saying Soviet leader Joseph Stalin would never had engaged in such a maneuver.
"Stalin was careful not to escalate things into a global war," Manchester University official Peter Lowe told the paper. "He felt the Soviet Union would not be ready for a world war until the mid-1950s. In any case, the magnitude of invading Japan and Taiwan would have been beyond the capabilities of the Soviet Union, China and North Korea."