Officials in Japan say little progress has been made in re-starting the cooling system at the stricken Fukushima Daiichi nuclear power plant.
The plant was badly damaged after a 9.0-magnitude earthquake triggered a tsunami that devastated the northeast coast of Japan on 11 March.
Workers have been battling to cool reactors and spent fuel ponds to avoid a large-scale release of radiation.
Engineers have managed to connect power cables to all six reactors at the Fukushima complex and started a water pump at one of them to reverse the overheating.
The BBC reports that on Monday, authorities were more upbeat about the operation to stabilise the power plant, but government ministers now say the situation is extremely tough and it is difficult to say that things are showing progress.
Steam and hazy smoke is continuing to rise from the plant on Tuesday, but radiation levels remain stable.
The Australian Air Force has delivered a water cannon to Tokyo. The remote-controlled device will be used to spray water into the reactors and help prevent serious exposure to radiation.
Until now, Japanese fire-fighters and soldiers have been risking contamination as they try to cool down the fuel rods.
Power rationing has started again after a three-day break for a long weekend. The nuclear shut-down is causing national shortages of electricity and fuel remains in short supply.
The head of the UN atomic agency says Japan's nuclear situation is still very serious but there is no doubt the country will effectively overcome the crisis.
International Atomic Energy Agency director Yukiya Amano said the agency's role in nuclear safety and standards may need to be re-examined. The IAEA draws up standards but can not enforce them.
Radiation levels in seawater and food
Abnormally high levels of radioactive material have been detected in seawater near the Fukushima plant.
Radioactive iodine levels are more than 100 times higher than government standards, while radioactive caesium are 25 times the official limit.
The World Health Organisation says radiation detected in food in Japan, such as vegetables and milk, is worse than previously thought.
The most toxic has been radioactive iodine and indications of radiocaesium.
Officials in the areas closest to the Fukushima plant have found higher than usual levels of radioactive iodine in samples of spinach and milk.
China and South Korea say they will toughen checks on Japanese food for radioactivity.
The WHO says it has no evidence of contaminated food spreading internationally.