UN chief Ban Ki-moon on Monday pledged the world body's solidarity with Japan after its quake, tsunami and nuclear disasters and encouraged radiation evacuees to "hang in there".

Travelling in the disaster-struck northeast, Ban became one of the most senior foreign leaders to visit the region close to the crippled Fukushima Daiichi atomic power plant, which is still leaking radiation.

"I came here to express my solidarity, the United Nations' solidarity for the government of Fukushima, and particularly for affected people in Fukushima," Ban told prefectural governor Yuhei Sato.

"Particularly this Fukushima Daiichi nuclear power plant accident has given us great lessons," he said. "We need to carefully review to improve our safety and improve our capacity tools in such an emergency response."

Some 85,000 people have fled the region around the plant after the world's worst nuclear disaster since Chernobyl 25 years ago.

Ban, who has convened a nuclear safety summit for the UN General Assembly in New York in September, is expected to reinforce his calls for tougher international standards.

Joined by his wife, Ban visited a shelter where more than 300 evacuees, mainly from Minamisoma city and a 20-kilometre (12-mile) no-go zone around the plant, have lived in cramped conditions for the past five months.

"You will hang in there," Ban said in Japanese to the evacuees, who live in tiny spaces separated by cardboard partitions.

Ban, who arrived in Japan Sunday, then visited Fukushima Minami High School, where he also gave words of encouragement to some 100 teenagers, telling them, "the entire world and the United Nations are behind you".

Five months on from the disaster, Japan's government and the plant operator Tokyo Electric Power Company (TEPCO) are struggling to stabilize three reactors at the plant following a series of meltdowns and explosions.

Japan wants to bring all reactors to stable "cold shutdown" by January.

But lethal hotspots were detected inside the crippled plant last week, with radiation so high that they threaten to prevent emergency workers from making progress in the effort to control the crisis.