Highly toxic plutonium is seeping from Japan's damaged Fukishima nuclear complex. Small amounts of the toxin were detected at several spots in the soil outside the facility.
Plutonium is a radioactive chemical element used to make nuclear weapons.
Safety officials say the amounts released are not a risk to humans, but support suspicions that dangerously radioactive water is leaking from the nuclear fuel rods.
The plant has been leaking radiation that has made its way into vegetables, raw milk and tap water as far away as Tokyo. Residents within 12 miles of the plant were ordered to leave, and some nations banned the imports of food products from the Fukushima region.
The tsunami spawned by the March 11 magnitude 9.0 offshore earthquake slammed into the plant and destroyed the power systems needed to cool the rods at the facility.
Experts called this latest development very grave.
"This is really an international nuclear crisis and it's far beyond the capability of one utility, being TEPCO, or even the Japanese government," said Naj Meshkati, an engineering professor with the University of Southern California.
On Tuesday, Prime Minister Naoto Kan told Japan's parliament that the country was grappling with its worst problems since World War II.
"This quake, tsunami, and the nuclear accident are the biggest crises for Japan in decades," said a wan but resolute Kan, dressed in one of the blue work jackets that have become ubiquitous among bureaucrats since the tsunami.
Kan said the crises remained unpredictable, but added: "We will continue to handle it in a state of maximum alert."
Meanwhile, the official death toll from the earthquake and tsunami has topped 11,000 and is expected to rise to 18,000. Hundreds of thousands remain homeless, their homes and livelihoods destroyed.
Damage could amount to $310 billion - the most expensive natural disaster on record, the government said