Tokyo Electric Power Co. said Tuesday it has detected radioactive cesium about 1 km off the coast of the crippled Fukushima No. 1 nuclear plant for the second time since it started the survey in August.

Cesium-137 was detected at a level of 1.6 becquerels per liter from seawater extracted Friday, higher than the reading measured earlier at the same survey point. But Tepco said it believes the impact on the environment is "little."

The latest data could further undermine Prime Minister Shinzo Abe's repeated remarks that the situation involving the buildup of massive radioactive water at the plant is "under control" and that the impact of radiation leaks is contained inside the plant's artificial harbor.

Seawater extracted Oct. 8 had a reading of 1.4 becquerels per liter of cesium-137.

The figures are lower than the legal limit for the release of cesium-137 from nuclear power plants, which is set at 90 becquerels per liter. They are also below the radiation level of the World Health Organization's safety standards for drinking water.

The volume of radioactive water is increasing daily at the nuclear plant because groundwater is seeping into reactor buildings and mixing with water used to cool the three stricken reactors.

Tepco has stored highly toxic water in hundreds of tanks set up at the site, but leaks have occurred from time to time. Some of the groundwater, which is flowing through the plant's premises toward the ocean, is also believed to be contaminated.