Greenpeace called on Tokyo to toughen radiation screening and food labelling rules on Thursday after it said low levels of radiation had been detected in seafood sold at Japanese stores.

The environmental pressure group said it tested 60 seafood samples bought at stores in eastern Japan operated by five major supermarket chains and found 34 of them with radioactive caesium-134 and caesium-137.

The survey discovered readings of up to 88 becquerel per kilogram with the radiation believed to be from the ongoing nuclear accident.

"While the samples are well below the 500 becquerel per kilogram limit set by the authorities, the contaminated seafood still represents a health risk, especially to pregnant women and children, and it is being distributed over a wide area," said Wakao Hanaoka, Greenpeace Japan oceans campaigner.

The Japanese standard compares with a 150 becquerel per kilogram limit in Ukraine after the 1986 Chernobyl disaster, the group said.

"More concerning, however, is that there is no labelling that notifies consumers if the seafood had been screened, making it impossible for them to make informed decisions," Hanaoka said in a statement.

The announcement came as Japanese consumers remain frustrated over limited information about the exact level of food contamination, while the government has sought to calm public fears and overcome mistrust of official radiation surveys.

Authorities say food is safe but consumers have generally avoided products from the regions near the wrecked Fukushima Daiichi nuclear plant, which went through meltdowns and explosions after the March 11 earthquake and tsunami.

The Fukushima disaster is the worst atomic disaster since Chernobyl 25 years ago.

Greenpeace, which bought the samples in September and October, has requested the government and a retail industry body to improve seafood product screening and to publicise the level of contamination in the labels.

The group has sent letters to the Japan Chain Stores Association demanding them to conduct radiation checks on their own and disclose the results to consumers.

Comprehensive screening and labelling should help protect consumers and raise their confidence in food they purchase, said Greenpeace's Hanaoka, adding that it should eventually help the fishing industry.

"Japanese consumers have legitimate concerns about the food they buy," Greenpeace's Hanaoka said.