Levels of radioactive substances have jumped in the Pacific seabed off Japan near the nuclear power plant crippled by a massive tsunami in March, according to the plant operator.

Seabed samples collected some 15 kilometres (nine miles) from the Fukushima Daiichi nuclear plant contained 1,400 becquerels of radioactive caesium-137 per kilogramme, Tokyo Electric Power Company (TEPCO) said.

The level is more than 600 times higher than a maximum 2.3 becquerels per kilogramme detected in the past off the northeastern prefecture of Fukushima.

The samples, taken on Friday, also contained 1,300 becquerels of caesium-134 and 190 becquerels of iodine-131, according to a TEPCO statement issued late Tuesday after the first analysis of seabed soil since the March 11 disaster.

Levels of the two materials were too low to be gauged in the past, a TEPCO spokeswoman said. The company did not say whether the levels were considered harmful.

Samples taken at another spot 20 kilometres (12 miles) away from the plant also showed similarly high radiation levels.

TEPCO said it would continue to examine radiation levels.

"We cannot say anything definite after just one probe. We will conduct more sample examinations and keep a watch," the spokeswoman said.

The samples were taken 20-30 metres (66-100 feet) deep and three kilometres from the coast.

The Fukushima plant has leaked radiation into air, soil and ocean since it was severely damaged by the massive quake and tsunami.

Greenpeace said Tuesday it had begun independent tests of water samples from the ocean near Japan's crippled for radiation contamination.

The samples will be collected outside Japan's 12-mile territorial waters in line with government rules, Greenpeace said.