The radioactivity of saltwater near the Fukushima nuclear power plant reached 7.5m times the legal limit last weekend but has since declined, its operator reported on Tuesday as Japan made a rare appeal for Russian help to cope with the contaminated waste.

The Tokyo Electric Power Company (Tepco) insisted that iodine, caesium and other radionuclides were being diluted to safe levels in the Pacific, but the firm's inability to stem the leak has prompted concern among fishermen, seafood consumers and neighbouring countries.

The data was taken from an area closer to the intake pipe of the cracked No 2 reactor than previous lower readings further out to sea. Publication was held back for several days at the insistence of nuclear safety authorities who wanted Tepco to check its numbers after an earlier botched release.

The latest figures show levels of radioactive iodine fell from 7.5m times the legal standard on Saturday to 5m times on Monday. Monitoring stations located several hundred metres off the coast also showed contamination has fallen to about 1,000 times the standard, compared with 4,000 last week.

Tepco said even the large amounts would have "no immediate impact" on the environment, though the chief government spokesman, Yukio Edano, has previously warned a prolonged leak could have a huge effect on marine life.

In the latest attempt to stem the radioactive flow, workers will inject "water glass" or sodium silicate on the floor of the cracked pit. Previous efforts involving cement, absorbent polymer and rags have been unsuccessful. Despite colouring the water with white bath salts, engineers have yet to trace the path of contamination.

In a sign of the company's desperation, Tepco breached its own regulations on Monday by starting an intentional discharge of 11,500 thousands of less contaminated water into the Pacific to make containment space for the more highly radioactive liquid that is seeping out uncontrolled.

South Korea's embassy in Tokyo has reportedly expressed concern at the pumping of water that is 100 times the legal limit, but was unclear at the effect it would have on its own waters.

"For now, we have no clear standards to determine how much is how bad for us," a foreign minister official was quoted as saying by the Yonhap news agency.

Environmental groups warn the radioactivity could enter the ocean food chain. Japan's industry minister, Banri Kaieda, said the discharge would not pose a major health risk, but apologised to fisherman.

"The discharge from unit 2 is incredible. This is clearly water contaminated by direct contact with the core fuel, which means it is likely to contain heavier isotopes, such as plutonium," said Shaun Burnie, nuclear adviser to Greenpeace Germany. "The release of long-lived radionuclides will have a severe impact on both public health and coastal economies. The plutonium will persist in coastal sediments effectively for ever."

According to Japanese media, fish have been found with traces of iodine and caesium in neighbouring Ibaraki prefecture. The government said it will strengthen monitoring systems and not allow a resumption of fishing until the situation is clear.

Tepco said on Tuesday it would pay provisional compensation around the end of April to farmers, local residents and other people affected by the accident.

The operation to stabilise the Fukushima plant is likely to take months. Kaieda said the basement of reactor buildings and underground trenches have been flooded with 60,000 tonnes of radioactive water that will have to be pumped into alternative vessels, including waste tanks, an artificial floating island and US navy barges.

Japan has requested assistance from Russia to deal with the huge and growing amount of contaminated material.

Sergei Novikov, a spokesman for the Russia's state-run Rosatom Corporation, told reporters on Monday that Japan has expressed an interest in a floating radioactive waste treatment facility. The Landysh, as the barge is named, was reportedly built with Japanese assistance.

Restoring public trust is also likely to take months.

Unease about reliability and transparency were fuelled by reports that the Meteorological Agency has withheld forecasts on the dispersal of radioactive substances. Although the government body provided information on wind patterns and discharges to the International Atomic Energy Agency, the Japanese public was kept in the dark. The meteorologists said they did not want to create a panic with simulations and forecasts that had a low level of accuracy. After criticism in the domestic media, the agency has pledged to release future forecasts.