Japan has made "significant progress" in cleaning up the crippled Fukushima nuclear plant, a UN review mission said Tuesday as it again advised the country to consider discharging treated water into the sea.

"Japan has made significant progress since our previous missions" in 2013, said Juan Carlos Lentijo, who led a review mission from the International Atomic Energy Agency.

"The situation on the site has improved -- progressive clean-up has led to reduced radiation levels in many parts of the site," he said in a statement as the agency ended a third mission.

The 15-strong IAEA team has been in Japan since last week to examine efforts by Tokyo Electric Power (TEPCO) to clean up the site, where reactors were sent into meltdown by a huge tsunami that crashed ashore in March 2011.

He called the successful removal of spent fuel rods from a storage pool a "milestone" and praised other efforts such as improved measures to clean contaminated water.

But the mission leader warned the situation "remains very complex" over the long term due to an increasing amount of contaminated water, the need to remove highly radioactive materials and the persistent ingress of groundwater.

Disposing of the thousands of tonnes of water which was used to cool reactors or polluted by other radioactive material is a major headache for TEPCO.

At present huge storage tanks are being used but there is no permanent fix.

The mission urged Japan to come up with a more "sustainable solution" and advised that it consider discharging treated water into the sea.

"The site is fortunately big but the space could run out if the situation continues for a long time," Lentijo told reporters.

While noting the UN agency was not advocating this particular measure, the team leader said controlled discharge was "something that is happening every day worldwide in most nuclear power plants" and had negligible impact on the environment.

Most experts agree that the water will eventually have to be released into the ocean after being scoured of its most harmful contaminants, but local fishermen, neighbouring countries and environmental groups all oppose the idea.

The roadmap towards the decommissioning of the Fukushima plant envisages a process that is likely to last three or four decades.

In its preliminary report issued Tuesday the IAEA also said it "strongly encourages TEPCO ... to reinforce safety leadership and safety culture" at the plant, where some 7,000 workers are engaged.

One man died there in January after falling into a water tank.

"There is still some room to enhance this interaction between radiation safety and labour safety through more integrated plans," Lentijo said.

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Tokyo Electric Power