A radiation hotspot has been detected in Tokyo, officials said Thursday, but authorities said it may not be linked to the crippled Fukushima nuclear plant.

As researchers carry out stringent tests to map how far contamination has spread from the plant, a mayor of western Tokyo told reporters that glass bottles found under the floorboards of a nearby house were likely the cause.

Local media had widely suspected that the hotspot was created after the March 11 disaster crippled the Fukushima Daiichi nuclear power plant, which has leaked radiation into the environment.

But as researchers inspected the area more closely, they found the levels were even higher inside a house adjacent to the sidewalk and discovered a box containing several glass bottles under the floorboards.

"We have identified the exact spot of high radiation," said Setagaya mayor Nobuto Hosaka at a late Thursday press conference.

"When posing a detector close to the bottles, the reading was so high that it was often impossible to measure it," he said.

"We are currently investigating the objects which are believed to be the cause of the high radiation levels."

No one reportedly lives in the house.

Researchers earlier found radiation levels of up to 3.35 microsieverts per hour along a street in the west of the capital -- 220 kilometres (136 miles) from Fukushima -- much higher than previously reported levels, officials said.

According to calculations based on the Japanese science ministry's criteria, the equivalent annual dose in the hotspot would be 17.6 millisieverts, just below the 20 millisieverts per year threshold that requires evacuation.

The inspectors found the levels near the glass bottles were more than 30 microsieverts, officials said.

Radiation fears are a daily fact of life in many parts of Japan following the earthquake and tsunami-sparked meltdowns at the plant, with reported cases of contaminated water, beef, vegetables, tea and seafood.

Variable winds, weather and topography result in an uneven spread of contamination, experts say, and radioactive elements tend to concentrate in places where dust and rain water accumulate such as drains and ditches.

The March 11 earthquake triggered a tsunami that tore into Japan's northeast coast, leaving 20,000 people dead or missing, while sparking meltdowns and explosions at the Fukushima Daiichi nuclear power plant.

The subsequent release of radiation forced the evacuation of tens of thousands from a 20 kilometre (12 mile) radius from the plant and spots beyond in the world's worst nuclear accident since the Chernobyl disaster in 1986.