The Nuclear Regulation Authority said Wednesday it strongly suspects highly radioactive water at the Fukushima No. 1 nuclear power plant is seeping into the ground and contaminating the Pacific Ocean.

"We must find the cause of the contamination . . . and put the highest priority on implementing countermeasures," NRA Chairman Shunichi Tanaka told a meeting of the body's commissioners after they had examined recent studies carried out on groundwater samples at the plant that detected high levels of cesium, tritium and other radioactive contamination.

Tokyo Electric Power Co. claims it believes the source of contamination to be a pit from which highly radioactive water was found seeping into the sea in April 2011, shortly after the nuclear crisis erupted at the plant. However, the NRA said Tepco's explanation was questionable and noted that toxic water in the area may not be the only source.