Radiation worries have disrupted efforts to restart the cooling system of the Fukushima Daiichi plant, which was battered by a huge earthquake and tsunami on March 11 that left more than 28,000 people dead or missing.
With Japan struggling to contain its worst ever atomic crisis, France said its nuclear groups Areva and EDF had been asked to help.
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Plant operator Tokyo Electric Power Company (TEPCO) said plutonium was found at five spots within the crippled facility, but stressed the levels were not believed to be a hazard to human health.
"Of the samples from five locations, we believe that there is a high possibility that at least two of them are directly linked with the current reactor accident," a spokesman for TEPCO said.
But he added: "We believe the level is not serious enough to harm human health."
The level of plutonium was similar to that detected in Japan after neighbouring countries such as North Korea and China conducted nuclear experiments, the spokesman said.
Japan's Nuclear and Industrial Safety Agency said it was unclear which reactor the plutonium came from.
Hidehiko Nishiyama, a spokesman for the agency, said the detection of plutonium suggested "certain damage to fuel rods", in comments carried by Kyodo News.
According to the US environmental protection agency, external exposure to plutonium poses "very little health risk", but internal exposure "is an extremely serious health hazard".
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, it said.
The massive earthquake and tsunami which crashed into northeast Japan knocked out the cooling systems for the six reactors of the Fukushima plant, which has been leaking radiation into the atmosphere.
The latest discovery will add to environmental concerns over contamination which has already been detected in farm produce and tap water, although Japan says there is no imminent health threat.
Traces of radioactivity from the Fukushima plant have been found in rainwater in the northeast United States, but pose no health risk, the US environmental protection agency said.
Earlier Monday TEPCO said a large amount of highly radioactive water had escaped from the number two reactor's turbine building into an underground tunnel and might leak into the ocean.
The water is thought to have leaked from the vessel containing the fuel rods - which are suspected to have temporarily melted - or from the pipe system.
The radiation in the water was measured at 1000 millisieverts an hour, a dose that can cause temporary radiation sickness with nausea and vomiting for people who are exposed.
Seawater near the plant has been found to contain radioactive iodine more than 1850 times the legal limit, although it is not exactly clear how the contamination spread to the Pacific Ocean.
TEPCO was severely reprimanded by the government yesterday, a day after it erroneously said radiation in water at the site had reached 10 million times the normal level before later issued a much lower - but still dangerous - figure.
"Considering the fact that the monitoring of radioactivity is a major condition to ensure safety, this kind of mistake is absolutely unacceptable," said top government spokesman Yukio Edano.
Adding to questions about the handling of the crisis, TEPCO said its president Masataka Shimizu took several days off from a joint emergency taskforce with the government due to sickness, but has now returned to work.
The group has also faced criticism over an incident last week in which two plant workers braving hazardous conditions were hospitalised because they stepped in radioactive water without proper boots.
TEPCO shares plunged nearly 18 per cent on the Tokyo Stock Exchange yesterday, while the broader market saw a day of subdued trading as the Nikkei 225 index slipped 0.6 per cent.
Work to restore power at reactor two has been suspended since Sunday because of the danger posed by the radioactive water leaks.
The immediate focus is on draining the highly radioactive water from the turbine room basements, but without releasing it into the environment.
The nuclear crisis remains a distraction from the dire plight of hundreds of thousands made homeless in the quake-tsunami tragedy.
The disaster, Japan's deadliest since the 1923 Great Kanto Earthquake, has left 10,901 dead and 17,649 missing, the National Police Agency said yesterday.
French President Nicolas Sarkozy will make a short visit to Japan during a scheduled trip to China this week, an official from his party told AFP yesterday, but the precise date is unclear.