Secretary for Environment and Ecology Tse Chin-wan said although the wastewater from the damaged Fukushima nuclear plant would be treated before discharging into the Pacific Ocean, any errors in the process would significantly affect ecology and food safety. The concern stems from Japan's U.N.-endorsed, but controversial, plan to gradually release the treated water.

"Our assessment shows prefectures near Fukushima have higher risks, so we are now taking a responsible way for our residents," he told reporters at a briefing.

The 10 affected territories are Tokyo, Fukushima, Chiba, Tochigi, Ibaraki, Gunma, Miyagi, Niigata, Nagano and Saitama, he added.

A massive earthquake and tsunami in March 2011 destroyed the Fukushima Daiichi nuclear plant's cooling systems, causing three reactors to melt and releasing large amounts of radiation. The tanks storing the water used since the accident to cool the reactor cores will reach their capacity in early 2024.

In 2021, Japan's government announced plans to gradually release the treated—but still slightly radioactive—water following its dilution to what it says are safe levels. Japanese officials say the water, currently stored in about a thousand tanks at the plant, needs to be removed to prevent accidental leaks in case of an earthquake and to make room for the plant's decommissioning.