Japan's government will clamp down on contractors cleaning up radioactive material around the ruined Fukushima nuclear plant, officials said Wednesday, following disclosures of sloppy decontamination work.

The Environment Ministry hired the nation's leading contractors to cleanse towns and villages near the tsunami-hit Fukushima Daiichi power plant, starting with four relatively uncontaminated areas.

But the Asahi Shimbun daily reported last week that dirty soil, leaves and water have been dumped directly into rivers. The paper cited workers as saying they were told to sweep only around radiation monitoring sites.

Senior Vice Environment Minister Shinji Inoue visited Fukushima on Wednesday and apologised to local residents.

The head of the ministry's special office in Fukushima admitted Monday that the authority had confirmed at least two cases in which dirty water was allowed to escape directly into the environment during decontamination work.

Water used to hose down buildings is supposed to be collected and sent for purification before it is released, while soil and leaves should be collected for storage.

The government has drawn up guidelines for workers, but the Asahi report quoted some as saying the decontamination project is so vast and painstaking that they would not finish in time if they followed the rules.

"The ministry ordered the contractors to investigate the situation and submit a report by Friday," said a ministry official, adding the government will draw up measures to deal with the problem over the following week.

The decontamination work is largely being carried out by four companies on contracts worth millions of dollars each.

In the town of Naraha, Maeda Corp's joint venture is carrying out the clean-up project in a deal worth 18.82 billion yen ($215 million), while Taisei Corp won a 7.72 billion yen contract for the work in Iitate village.

The government has allocated nearly one trillion yen ($11.4 billion) to implement a special law introduced to cope with the serious nuclear contamination. The cost is expected to rise.

A massive earthquake and tsunami on March 11, 2011 devastated Japan's northeast coast and triggered the worst atomic crisis in a generation, forcing tens of thousands to evacuate the area surrounding the plant.

No one is officially recorded as having died as a direct result of the nuclear meltdowns, but thousands have been made homeless.

The clean-up around Fukushima is expected to take decades and experts warn that some settlements may have to be abandoned.