Japan's efforts to clean up its nuclear disaster face intense global scrutiny ahead of the Tokyo Olympics, observers say, but despite government promises that Fukushima is "under control" the crisis will not be over by 2020.

Speaking to Olympic chiefs in Buenos Aires just ahead of their weekend decision to award the Games to Tokyo, Japanese Prime Minister Shinzo Abe said there was nothing to worry about at the plant.

"Let me assure you, the situation is under control," he said in a speech lauded by Japanese media as key to Tokyo's success.

"It has never done and will never do any damage to Tokyo."

"Contaminated water has been contained in a 0.3 square-kilometre area of the harbour," he added in a question-and-answer session.

"There have been no health problems and nor will there be. I will be taking responsibility for all the programmes with regard to the plant and the leaks."

Critics at home and abroad say Abe's gloss on the disaster at Fukushima, where a tsunami swamped cooling systems and sent reactors into meltdown, is bordering on the dishonest.

"I was flabbergasted by Abe's speech," said Hiroaki Koide, an associate professor at Kyoto University Research Reactor Institute.

"The problem of contaminated water is far from being solved. This problem has been going on all the time since the reactors were destroyed. Contaminated water has been leaking into the ocean ever since."

Late Monday, plant operator Tokyo Electric Power reported spiking levels of radiation in groundwater and said it was "likely" leaks from tanks storing highly polluted water had made their way into subterranean water, further complicating efforts to stem pollution.

Groundwater flows out to sea, taking along anything it has picked up and dumping it in the ocean.

Tomoo Watanabe, director of the Research Center for Fisheries Oceanography and Marine Ecosystems, said his understanding of the situation at Fukushima is not that it is "contained" in the way Abe explained it.

But he said he agreed with the prime minister that it is necessary to look behind the alarming headlines to see the the truth.

"You may have a definite impression that the ocean is much more contaminated after TEPCO admitted to the water leak, but we have not seen any signs of that pollution spreading to fish," he told AFP.

Around 300 tonnes of mildly contaminated groundwater is entering the ocean every day, TEPCO says, having passed under the reactors.

Watanabe said fish caught offshore -- outside the harbour -- have shown a gradually decreasing level of caeseum contamination, more markedly so in waters 20 kilometres (12 miles) from the plant.

But, he added, the pollution inside the harbour is high and fish living there should not be allowed to escape into the ocean, where they would enter the food chain.

After weeks of bad news from Fukushima and amid a rising clamour of international criticism, Japan's government stepped in last week with a half-billion dollar plan aimed at stemming the flow of polluted water reaching the sea.

Critics point out that so far, much of the work done at Fukushima to stabilise the plant has been temporary -- the tanks storing highly radioactive water used to cool overheating reactors were never intended to be a permanent solution.

TEPCO's own estimates suggest the full decommissioning of the site could take up to four decades and that much of the trickier work is yet to be done -- notably the removal of reactor cores that have probably melted beyond recognition.

According to the utility's own plan, these reactor cores -- which are feared to have seeped into the containment vessels and possibly even eaten through thick concrete -- will be removed around summer 2020, just as thousands of athletes descend on Tokyo.

Hiroshi Miyano, a nuclear plant expert and visiting professor at Hosei University in Tokyo, said despite the niceties in Buenos Aires, the clean-up was still a tall order.

"The Olympic success may give positive momentum and speed up the roadmap, but I'm afraid it will still take at least two decades to decommission Fukushima at best."

In an editorial published Tuesday, the left-leaning Asahi Shimbun said the fact of Shinzo Abe's having stood on an international stage and promised to resolve the Fukushima crisis was a welcome move.

"This is an official pledge made to the world," it said.

"Abe must take action so that he won't be seen at home and abroad to have just stretched the truth to bring the Olympic Games to Tokyo. His ability to address this issue is now being watched."