North Korea on Sunday slammed the United Nations' nuclear watchdog for approving Japan's plan to discharge treated water from the Fukushima nuclear plant.

The International Atomic Energy Agency approved Tokyo's plan to release treated water from the tsunami-hit nuclear plant into the sea over the next few decades.

The plan has raised concerns in neighbouring countries, prompting China to ban some food imports and sparking protests in South Korea.

The release of the treated water will have a "fatal adverse impact on the human lives and security and ecological environment," an official from Pyongyang's environmental protection ministry said in a statement carried by the official Korean Central News Agency.

"What matters is the unreasonable behaviour of IAEA actively patronizing and facilitating Japan's projected discharge of nuclear-polluted water, which is unimaginable," the statement added.

Some 1.33 million cubic metres of groundwater, rainwater and water used for cooling have accumulated at the Fukushima nuclear plant, where several reactors went into meltdown after the 2011 tsunami overwhelmed cooling systems.

The plant operator treats the water to remove almost all radioactive elements except tritium, and plans to dilute it before discharging it into the ocean over several decades.

The statement comes as IAEA head Rafael Grossi is wrapping up his three-day visit to Seoul with a meeting with opposition lawmakers, who have criticised the planned release.

On Saturday, Grossi met with South Korean foreign minister Park Jin to brief him on his agency's findings, Seoul's foreign ministry said in a statement.

Park requested "active cooperation from the IAEA for safety verification and public reassurance," it added.

Following that meeting, Grossi said the IAEA will remain at the Fukushima plant to ensure safety "every step of the way", tweeting: "What starts now is even more important than the work done so far - the continuous monitoring of the plan's implementation."