The US army tested biological weapons that could harm rice cropping on the Japanese island of Okinawa in the early 1960s, a report claimed yesterday.
The same experiments were conducted on the US mainland and in Taiwan, Kyodo news agency said, citing US military documents.
The US is "believed to have had China and Southeast Asia in mind in developing such crop-harming agents", the report said.
In tests conducted at least a dozen times between 1961 and 1962, rice blast fungus - known to occur in 85 countries - was released over rice fields and data was collected on how it affected rice production, Kyodo said.
Rice blast disease causes lesions to form on the plant, threatening the crop.
The US government decided in 1969 to discard all biological weapons in its possession, Kyodo said. In 1975 an international convention against production and possession of biological weapons came into force.
Okinawa was under post-second world war US rule until 1972. The US government has previously disclosed information about chemical and biological warfare tests at sea and on land in such places as Puerto Rico, Hawaii and Utah.
The obtained documents mention test sites including Nago and Shuri, both in Okinawa, but it is not known whether the experiments were conducted within the premises of US bases there, Kyodo said.
In the tests, the army "used a midget duster to release inoculum alongside fields in Okinawa and Taiwan", measuring dosages at different distances and the effect on crop production.
One document said: "Field tests for stem rust of wheat and rice blast disease were begun at several sites in the [US] midwest and south and in Okinawa with partial success in the accumulation of useful data."