The International Atomic Energy Agency (IAEA) will play a central and "permanent" role in monitoring Japan's discharge of waste water, which has been treated but is still contaminated with radioactive tritium, from the crippled Fukushima No. 1 nuclear plant into the sea, Director General Rafael Grossi said in an interview Wednesday.

He also said that the U.N. nuclear watchdog's involvement, including dispatches of experts, would lend credibility and offer reassurance about environmental safety.

Japan's insistence that the water does not pose a danger to health is disputed by neighboring countries as well as domestic fishing groups.