The ministry said radioactive cesium of 500 becquerels per kg was found in a sample of the pre-harvested rice in Nihonmatsu city, in Fukushima Prefecture, 56 km (35 miles) west of the Fukushima Daiichi nuclear plant which was crippled by the March 11 earthquake and tsunami, triggering the world's worst nuclear disaster in 25 years.

The ministry said the Fukushima Prefecture will expand the inspection spots nearly ten-fold to around 300 areas.

It is the first case in Japan of rice containing radioactive cesium exceeding 200 becquerels per kg, a level which requires further thorough testing of the area for the harvested rice.

The government introduced inspection guidelines in August, with preliminary tests followed by more before approving shipments.

If preliminary tests found rice to contain radioactive cesium levels of 200 becquerels per kg or more, the crop will be tested more thoroughly before approvals are made for shipments.

If the level of cesium in rice exceeded the government-imposed cap of 500 becquerels per kg, shipments from locally produced rice will be halted.

So far, no rice crop has been banned for shipments.

If the follow-up tests of rice harvested from Nihonmatsu city find radioactive materials exceeding the government-imposed cap, it would deal a huge blow to Japan.

The country has been struggling to regain public trust in the safety of nuclear power so it can resume operations of nuclear reactors to supply energy as well as food safety after wide-ranging products from water to vegetables were found with radiation contamination.