The Tokyo Electric Power Company (Tepco) said it had injected chemical agents to solidify soil near a cracked pit, from where the contaminated water had been seeping out.

Engineers have been struggling to stop leaks since the plant was damaged by the earthquake and tsunami on March 11.

Japan has asked Russia for the use of a floating radiation treatment plant.

In another development, government sources said that a plan to cover the damaged reactor buildings with special metal sheets could not be carried out until September at the earliest due to high-level radioactivity hampering work at the site.

The official death toll from the 9.0-magnitude earthquake and tsunami stands at more than 12,000 with some 15,000 people still unaccounted for, and more than 161,000 people still living in evacuation centres.

Samples of water used to cool one of the plant's six reactors, No 2, showed 5m times the legal limit of radioactivity, officials said on April 5.

Russia's nuclear agency Rosatom said it was awaiting answers to some questions before granting Japan's request to lend its vessel the Landysh, known in Japanese as the Suzuran, which is used to decommission Russian nuclear submarines in the far eastern port of Vladivostok.

One of the world's largest liquid radioactive waste treatment plants, the Landysh treats radioactive liquid with chemicals and stores it in a cement form.

It can process 35 cubic metres of liquid waste a day and 7,000 cubic metres a year.