On Friday, Foreign Minister Katsuya Okada was notified of the discovery by the 15-member team, which found the files while examining ministry documents.
“The probe is now in the final stage, and we will announce the outcome in January,” Okada said, Kyodo News reported on Saturday.
Nuke-laden American Navy warships and Air Force aircraft were sanctioned by the 1960 deals to enter the Japanese waters. The clandestine understanding has already been pointed to in US documents and confirmed by those involved in the negotiations.
A former Japanese official who served in key ministry posts in the 1980s and 1990s has also confirmed the existence of the document.
”I saw them. I have memories that we looked into them after something happened,” the former official told Kyodo on condition of anonymity.
The two sides are already at odds over Japan’s threats of removing a US air base from its southern island of Okinawa.
Some of the secret deals are reportedly related to the 1972 handover of the control of Okinawa from the US to Japan.