The U.S. government told Japan in 1970 it was storing chemical arms in Okinawa out of the belief that North Korea possessed similar weapons, according to declassified Japanese diplomatic documents released Thursday.

A secret cable dated Sept. 14, 1970, shows that when Defense Agency chief Yasuhiro Nakasone met U.S. Defense Secretary Melvin Laird that day in the United States, Laird said poison gas was brought into Okinawa in 1962 based on the recommendation of Robert McNamara when he was secretary of defense.

Laird explained to Nakasone the United States had information that North Korea had a supply of chemical weapons and that the U.S. stockpile in Okinawa was meant to serve as a deterrent, according to the cable.

He went on to say that since the U.S. no longer believed North Korea possessed such arms, the chemical weapons would be transferred from Okinawa to Johnston Atoll about 1,400 km southwest of Hawaii.

More than 13,000 tons of poisonous substances, including mustard gas and sarin nerve gas, was reportedly transferred to Johnston between January and September 1971.

The U.S. handed control of Okinawa back to Japan in 1972.

In July 1969, there was a chemical weapons leak at Chibana Ammunition Depot, which is near Kadena Air Base. The depot is now integrated into what is known as the Kadena Ammunition Storage Area.

The U.S. subsequently admitted it had chemical arms in Okinawa and two years later local residents succeeded in having it removed.