Tokyo (CNN) -- An extraordinarily high level of radiation was detected in one spot in a central Tokyo residential district Thursday, prompting the local government to cordon off the small area, local officials said.
Radiation levels were higher in Tokyo's Setagaya ward than in the evacuation area around the badly damaged Fukushima Daiichi nuclear plant, according to ward Mayor Nobuto Hosaka.
"We are shocked to see such high radiation level was detected in our neighborhood. We cannot leave it as is," Hosaka told reporters.
But the tsunami-struck Fukushima plant may not be the source of the radiation, Hosaka said later on state television.
Officials searching for the cause found "glass bottles in a cardboard box" in the basement of a house in the neighborhood which sent radiation detectors off the charts, he said on NHK.
"We suspect these bottles in basement could be the cause of the high radiation reading and we are hastily working to confirm it," he said.
Radiation experts are now checking what contaminated the bottles, a Setagaya ward official told CNN, declining to be named in line with policy.
They told the local government there are no immediate health hazards.
Radiation levels just a few feet from the contaminated spot are normal, Hosaka said.
The Tokyo scare comes a day after officials in Yokohama, Japan's second largest city, investigated soil samples after a radioactive substance was found in sediment atop an apartment building, according to news reports.
Yokohama is about 155 miles (250 kilometers) from the Fukushima No. 1 nuclear plant.
The discovery raised concerns that leaked radiation from three Fukushima reactors that suffered meltdowns after the March earthquake and tsunami may be more widespread than thought, The Japan Times reported Wednesday.
The U.S. government issued a travel alert last week, warning Americans in Japan to avoid areas near the stricken reactors.
The alert recommends that U.S. citizens stay away from areas within 20 kilometers (12 miles) of the nuclear facility. The State Department also admonished Americans to stay away from territory northwest of the plant in a zone that Japan calls the "Deliberate Evacuation Area." The zone includes Iitate-mura, the Yamagiya district of Kawamata-machi, Katsurao-mura, Namie-machi and parts of Minamisoma.
CNN's Yoko Wakatsuki contributed to this report.