In concentrated areas in Tokyo and Yokohama, Japanese researchers have found high levels of radioactive material.
The recorded airborne radiation levels in Tokyo were about 50 times higher than another location in Setagaya where the ward regularly monitors radiation levels.
"What's puzzling is that the levels detected on other parts of the same sidewalk were very low," the Wall Street Journal quoted Ken Hatanaka, head of the ward's section in charge of radiation monitoring, as saying.
He said Setagaya ward is consulting experts to figure out what to do with the highly contaminated spot, noting that rainwater and sediments containing radioactive fallout may have been more likely to accumulate in such spots.
Officials said the discoveries of such isolated, tiny 'hot spots' didn't indicate any immediate danger for residents of Japan's largest cities.
In Yokohama, the local government earlier detected 40,200 becquerels of radioactive cesium per kilogram of sediment collected from one part of a roadside ditch.
The city is investigating another spot on an apartment rooftop where tests conducted by the Isotope Research Institute detected over 60,000 becquerels of radioactive cesium per kilogram of sediment.
The discoveries come as Japanese researchers, local governments, and residents are stepping up their monitoring efforts.