Japan has revealed radiation up to several hundred times normal levels has been detected on the seabed off the crippled Fukushima nuclear plant, a report said Saturday.

The science ministry announced late Friday highly radioactive materials were detected in a 300-kilometre (190-mile) north-south stretch from Kesennuma in Miyagi Prefecture to Choshi in Chiba Prefecture, the Kyodo news agency reported.

The ministry warned that the contamination could affect the safety of seafood, the report said, without giving figures for the radiation levels detected.

The science ministry said it detected iodine and caesium on the seabed at 12 locations 15 to 50 kilometres from the coastline between May 9 and 14.

The news follows an announcement by environmental activist group Greenpeace on Thursday that marine life it had tested in waters more than 20 kilometres off the Fukushima nuclear plant showed radiation above legal limits.

The anti-nuclear group, which conducted the coastal and offshore tests this month, criticised Japanese authorities for their "continued inadequate response to the Fukushima nuclear crisis" sparked by the March 11 quake and tsunami.

Greenpeace said it detected seaweed radiation levels 50 times higher than official limits, which it charged raised "serious concerns about continued long-term risks to people and the environment from contaminated seawater".

It also said that tests, which it said were independently verified by French and Belgian laboratories, showed above-legal levels of radioactive iodine-131 and caesium-137 in several species of fish and shellfish.

In the aftermath of the quake small amounts of radiation from Fukushima spread across Asia, deepening concerns for millions of people in countries which had already imposed bans on Japanese produce from near the nuclear plant.

The governments of China, South Korea, the Philippines and Vietnam reported that radiation had drifted over their territories, although they emphasised the levels were so low that there was no health risk.

Fukushima prefecture has told AFP that no fishing is going on at the moment in its waters.

Japan's fisheries agency, and neighbouring prefectures, have been checking marine products at different spots, and the government has prohibited fishermen from catching some species found to have elevated radiation levels.

Hong Kong said Friday it had detected a small amount of radioactive iodine-131 in a sample of grey mullet but it was well below government limits.

The Hong Kong government did not say whether the iodine could be traced to the Fukushima plant.