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 can not 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 .
NEW : There is no immediate radiation threat , experts tell local officials . Local police cordons off an area in the Setagaya ward . Radiation there is higher than around Hiroshima . The damaged Fukushima nuclear plant could not be the source , experts say .