Japan said on Friday it finally had control of leaking reactors at Fukushima, in what authorities say is a vital step on the long road to recovery, nine months after its nuclear crisis began.

In a live press conference, Prime Minister Yoshihiko Noda told the country the crippled reactors 'have reached a state of cold shutdown'.

The government is hoping the announcement will bring relief to a disaster-weary public still haunted by the effects of the monster tsunami that tore into Japan in March.

Stabilisation of the reactors, whose molten cores spewed radioactive particles into the air and sea, marks the end of what the government has dubbed 'Step Two' of the nuclear clean-up.

The initial success of 'Step One' - the stable cooling of reactors and used fuel pools - was announced in July, after the quake-triggered tsunami pummelled the plant on March 11 and laid waste to much of the northeast coast.

"Today we have reached a major turning point with regard to the nuclear accident," said the prime minister.

But Noda warned the battle after the world's worst atomic accident for a generation was far from over.

Tens of thousands of people were evacuated from their homes in a large area around the plant as it began leaking cancerous isotopes in the days after March 11.

Swathes of this zone remain badly polluted, with the clean-up proceeding slowly amid warnings that some towns could be uninhabitable for three decades.

Noda said on Friday that he was ordering decontamination teams into the area.

"The government... will allocate more than one trillion yen ($13 billion) (and will) secure more than 30,000 workers who will do actual decontamination work by April."

Yuhei Sato, the governor of Fukushima prefecture, who has been an outspoken critic of Tokyo's response to the disaster, said cold shutdown changed little.

"I wish this was a new step toward helping evacuated residents go home. But there is no change in that. There is a long and rocky road ahead toward the resolution of the accident," he said.

Takashi Sawada, vice chairman of the Atomic Energy Society of Japan, a pro-nuclear group of academics and industry specialists, said Noda's declaration of cold shutdown was not a dramatic shift.

Sawada stressed that the use of the term 'cold shutdown' did not indicate that all four disaster-hit reactors were now completely normal.

"But I think it's okay to say that the reactors have basically reached a stable condition of cooling," he said, adding the amount of radiation leaking from the plant was now a tiny fraction of what it was in March.

Environmental campaign group Greenpeace dismissed the announcement as a 'smokescreen'.

"By triumphantly declaring a cold shutdown, the Japanese authorities are clearly anxious to give the impression that the crisis has come to an end, which is clearly not the case," said Junichi Sato, executive director of Greenpeace Japan.

"Instead of creating a PR smokescreen... the government’s priority should be to ensure public safety and begin the shutdown of all nuclear reactors in Japan."

"The ongoing radiological threat posed by the Fukushima nuclear disaster remains enormous."

While the natural disaster claimed 20,000 lives, the nuclear emergency has recorded no direct casualties. But it has badly dented the reputation of a technology on which Japan previously depended for a third of its electricity.

Waves up to 14 metres (45 feet) high swamped the reactors' cooling systems, sparking meltdowns, explosions and the release of radioactive material in the world's worst nuclear accident since Chernobyl in 1986.

Operator Tokyo Electric Power (TEPCO) was caught short by the disaster, with its tsunami defence systems overwhelmed and back-up power generators knocked offline.

That left a small band of men - dubbed the Fukushima 50 - to try a series of ad hoc solutions, including the use of seawater to cool the melted fuel rods.

This then-contaminated cooling water subsequently became a major headache for TEPCO, which had to release tonnes of it into the Pacific, provoking the ire of fishermen as far away as China.

Farmers in the area also suffered, with produce shunned by consumers or banned by the government because of radioactive contamination.

Sawada at the Atomic Energy Society said another big quake or tsunami could undo the hard work at Fukushima, and stressed that decommissioning the reactors and cleaning up the surrounding area would last decades.