Three independent U.N. human rights experts on Thursday expressed deep regret over Japan's recent decision to release treated radioactive water accumulated at the crippled Fukushima Daiichi nuclear power plant, saying it could impact millions of people across the Pacific region.

A joint statement issued by the three called Tokyo's decision "very concerning" and questioned its claim that the tritium levels in the water stored in tanks at the nuclear plant do not pose a threat to human health.

Photo taken from a Kyodo News helicopter in February 2021 shows tanks storing treated radioactive water on the premises of the crippled Fukushima Daiichi nuclear power plant. (Kyodo)

"The release of one million tons of contaminated water into the marine environment imposes considerable risks to the full enjoyment of human rights of concerned populations in and beyond the borders of Japan," the statement said.

The three special rapporteurs -- Marcos Orellana, Michael Fakhri and David Boyd -- said Japan's decision earlier this week is "particularly disappointing as experts believe alternative solutions to the problem are available."

"We remind Japan of its international obligations to prevent exposure to hazardous substances, to conduct environmental impact assessments of the risks that the discharge of water may have, to prevent transboundary environmental harms, and to protect the marine environment," the statement said.

Tokyo's move to discharge the treated water from the Fukushima nuclear plant, which was devastated by a massive tsunami following the magnitude-9.0 earthquake in March 2011, drew strong criticism from neighboring countries such as China and South Korea, while the United States showed understanding of the plan and the International Atomic Energy Agency supported it.