Japan's prime minister is insisting his country is on "maximum alert" to bring its nuclear crisis under control as officials race to stabilize an earthquake-damaged reactor complex and contain the spread of radioactive water.

Naoto Kan told parliament on Tuesday, before a new earthquake was recorded off Japan's main island of Honshu, that Japan was grappling with its worst problems since the Second World War.

"This quake, tsunami and the nuclear accident are the biggest crises for Japan" in decades, said Kan.

He said the situation remained unpredictable, but added: "We will continue to handle it in a state of maximum alert."


Japanese Prime Minister Naoto Kan reacts during the Upper House budget committee in Tokyo on Tuesday. Kyodo News/Associated Press

Tuesday's tremor, with a preliminary magnitude of 6.3, struck around 8 p.m. local time near the spot of the devastating March 11 quake, the U.S. Geological Survey reported. There were no immediate reports of damage.

The prime minister's comments also came as officials raised the alarm about the spread of radiation near the crippled Fukushima Daiichi nuclear power complex, 220 kilometres northeast of Tokyo.

Plutonium has been found in seawater and soil near the complex and some samples can be traced to the earthquake-damaged facility, plant operator Tokyo Electric Power said on Monday.

Officials said it appeared two of the five samples came from the plant.

A delicate balance

Radioactive water has been discovered in numerous places around the Fukushima nuclear complex. It must be pumped out and safely stored before electricity can be restored to the cooling system.

At the same time, however, water must also be pumped in to keep the reactor fuel rods cool.

Hiehike Nishiyama, a spokesman for Japan's Nuclear and Industrial Safety Agency, has called the sometimes-contradictory efforts "very delicate work."

Source: The Associated Press

TEPCO vice-president Sakae Muto said, however, the plutonium 238, 239 and 240 collected were not in concentrations harmful to human health.

The radioactive material is a byproduct of uranium-based power generation and is a known carcinogen.

Experts believe the plutonium found may have come from spent fuel rods at Fukushima or damage to reactor No. 3, the only unit at the six-reactor complex to use the material in its fuel mix.

Scientists are still trying to devise a plan to safely store the radioactive water seeping from the coastal plant.

It has been leaking radiation since a magnitude-9.0 quake on March 11 triggered a tsunami that engulfed the complex. The wave knocked out power to the system that cools the dangerously hot nuclear fuel rods.

Police said more than 11,000 bodies have been recovered, but the final death toll is expected to exceed 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.

With files from The Associated Press