A Tokyo radiation hotspot was not linked to the Fukushima nuclear crisis, Japanese authorities concluded Friday after finding old containers of radium powder likely used for luminous paint.

As researchers carry out more stringent tests to map how far contamination has spread from the crippled Fukushima atomic power plant, local media had widely suspected the radiation hotspot was created by fallout from the plant.

But researchers inspecting the area and a nearby house discovered a decaying wooden box containing several glass bottles under the floorboards, where the radiation levels hit more than 30 microsieverts per hour -- an alarming level.

"Radium 226 was found in the bottles, and it is not an isotope created in uranium fission. This hotspot has nothing to do with the Fukushima nuclear accident," said an official from the science and technology ministry.

"Some of the bottles were bearing signs indicating they were used for luminous painting, but it's difficult to identify what they were for exactly. They are so old."

The hotspot in western Tokyo was first discovered by a civil group measuring radiation levels in the capital, 220 kilometres (136 miles) from Fukushima, amid concerns about the spread of radiation from the crippled nuclear plant.

On Thursday researchers had found radiation levels of up to 3.35 microsieverts per hour in the vicinity of the site, before the bottles were found.

Based on the Japanese science ministry's criteria, that is equivalent to an annual dose of 17.6 millisieverts. A public evacuation is mandated if the figure reaches the equivalent of 20 millisieverts per year.

"No one lives in the house now, and just walking by near the house would have posed no health impact on neighbours," the official said.

Radium 226 is also used for medical purposes, he said.

But the origin of the bottles will likely remain a mystery, as he said the owner of the house and recent residents were unaware of it.

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

National Broadcaster NHK on Friday reported that a citizens' group in Funabashi City in Chiba, east of Tokyo, had detected radiation levels of up to 5.82 microsieverts per hour at a local park, compared to official readings of 1.55 microsieverts per hour at the site.

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, and 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.