During its inspection, the International Atomic Energy Agency will consult experts including from China and South Korea, which have reacted angrily to the release plan.

More than a million tonnes of processed water has accumulated in tanks at the crippled plant since it went into meltdown following a tsunami in 2011, including liquid that was used to cool damaged reactors.

An extensive pumping and filtration system removes most radioactive elements, and Japan says the plan to dilute and release the water over several decades is safe.

The IAEA has endorsed the release, which it says is similar to the disposal of wastewater at nuclear plants elsewhere.

"The review includes several missions and technical visits in coming months and years," Lydie Evrard, the IAEA's deputy director general, said Thursday in Tokyo on a visit to kick off the inspection process.

Before sharing the results, the IAEA will ensure its review is "comprehensive" and "objective", she said in an online briefing after meeting officials in Fukushima and the capital.

The Japanese government's decision in April to go ahead with the release -- which could begin as soon as March 2023 -- sparked ire from neighbouring countries over environmental and safety concerns.

It also generated fierce opposition from local fishing communities, who fear it will undermine years of work to restore confidence in their seafood.

Debate over how to handle the water has dragged on for years, as space to store it at the site runs out.

The filtration process removes most radioactive elements from the water, but some remain, including tritium.

Experts say the element is only harmful to humans in large doses and with dilution the treated water poses no scientifically detectable risk.

The IAEA will send future missions to review "the radiological characterisation" of the water, as well as how to release it and its impact on the environment and people, the Japanese industry ministry said.

Last month, plant operators unveiled plans to construct an undersea tunnel for the release of the 1.25 million tonnes of treated water, which also includes rain and groundwater that seeps in daily.