A limited amount of plutonium has been detected in soil outside Japan's troubled Fukushima nuclear power plant which was crippled by the March 11 quake-tsunami disaster, the government said Friday.

It was the first time plutonium had been found in government tests outside the plant, presumably due to the nuclear accident, the worst since 1986 Chernobyl, the education and science ministry said in a statement.

Plutonium was detected in soil at six places in a survey which was conducted in June in an area within 80 kilometres (50 miles) from the Fukushima Daiichi plant, the ministry said.

Nuclear reactors at the plant suffered meltdowns after cooling systems there were knocked out by the double disasters. Plutonium has been already detected in the plant's compound, some 220 kilometres from Tokyo.

The highest density of plutonium-239 and 240 -- 4.0 becquerels per square metre -- was registered in a town some 30 kilometres from the plant, the ministry said.

In a village 45 kilometres away, the reading was 0.82 becquerels per square metre.

Plutonim has previously been detected in Japan following atmospheric nuclear tests, the ministry said.

The average density of plutonium, which was detected in soil samples between 1999 and 2008 in Japan, was 0.498 becquerels, the ministry said. The highest reading before the Fukushima accident was 8.0 becquerels.

"The plutonium density, which was detected this time, was within the range of past readings. So the dose of radiation is deemed very small," the ministry said.

Plutonium is formed from uranium in nuclear reactors and generally stays in the body for decades, exposing organs and tissues to radiation and increasing the risk of cancer, experts say.

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