Radioactive cesium above government safety limits has been detected in rice near Fukushima, Japan, reports say.

The BBC reported the rice was found in a sample harvested in a district about 60 kilometres west of Fukushima City, and that none of the rice made it to market.

The rice came from an area outside the exclusion zone around the Fukushima nuclear plant, which was damaged in the March 11 earthquake and tsunami.

Japanese broadcaster NHK reported Fukushima prefecture has asked all 154 farmers in the district to suspend rice shipments.

NHK said government officials have interviewed 86 farmers in the district, leading them to estimate that one tonne of rice was sold to local dealers. The government is trying to determine if any of that rice made it to consumers.

The biggest earthquake to hit Japan since officials began keeping records in the late 1800s struck off the country's northeast coast March 11. The quake triggered a tsunami that swallowed homes, swept away cars and boats, and forced people to scramble to higher ground.

The earthquake and tsunami damaged the Fukushima Daiichi nuclear power plant, knocking out power to its cooling systems. That affected the fuel rods in some of the plant's six reactors and in the pools where spent but still-radioactive fuel rods are stored, setting off a series of hydrogen explosions that caused further damage.

The Japanese government boosted the severity level of the crisis to seven, the highest rating on the International Nuclear and Radiological Event Scale and on par with the 1986 Chornobyl disaster.