Lawmakers from South Korea's largest opposition party have told the chief of the UN nuclear watchdog of their concerns about Japan's plan to release treated and diluted water from the crippled Fukushima Daiichi nuclear plant into the ocean.

International Atomic Energy Agency Director General Rafael Grossi met officials of the Democratic Party on Sunday in Seoul.

Grossi reiterated that the IAEA's review of Japan's release plan is "in conformity with international safety standards." He added that he will carefully address the concerns of South Koreans.

The opposition lawmakers criticized the IAEA's inspection, saying it was biased in favor of Japan. They described the treated water as "radioactive waste."

South Korea's ruling party has accused the opposition of fanning fear with unscientific claims.

Water used to cool molten nuclear fuel at the Fukushima Daiichi plant mixes with rain and groundwater. The accumulated water is treated to remove most radioactive substances, but still contains tritium.

Before releasing the water into the ocean, the Japanese government plans to dilute the water to reduce the level of tritium to about one-seventh of the World Health Organization's guidelines for drinking water quality.

Grossi wraps up his three-day visit to South Korea on Sunday. He will then travel to New Zealand and the Cook Islands to discuss the IAEA's report.