The US army conducted field experiments of biological weapons, which could harm rice cropping, in the Japanese island of Okinawa in the early 1960s, a press report said Sunday.

The same experiments were also conducted on the US mainland and in Taiwan, Kyodo news agency reported, citing US military documents it said it had obtained.

The US is "believed to have had China and Southeast Asia in mind in developing such crop-harming agents", the report stated.

In the tests conducted at least a dozen times between 1961 and 1962, rice blast fungus was released over rice fields and data was collected on how it affected rice production, Kyodo said, citing the documents.

Rice blast disease causes lesions to form on the plant, threatening the crop.

The fungus, which is known to occur in 85 countries, is estimated to destroy enough rice to feed 60 million people each year.

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-World War II 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 field 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, the documents said.

A separate 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," according to Kyodo.