Japan has raised the incident level at its disaster-hit nuclear power plant, the International Atomic Energy Agency (IAEA) says.
The crippled Fukushima Daiichi site now has a level 5 rating, up from level 4 previously, on a 1 to 7 scale.
That would suggest a level of seriousness on par with the Three Mile Island accident in the United States in 1979.
Japanese engineers are toiling to avert a catastrophic release of radiation from the crippled plant 240km north of Tokyo.
They are trying to connect a power cable to at least two of the six reactors in the hope of restarting water pumps that spray the fuel rods with coolant.
Military helicopters have been dumping seawater on some of the reactors and military fire trucks have directed jets of water on them.
The IAEA, the United Nations nuclear watchdog, says it will start measuring radiation levels in Tokyo straight away.
It says it hopes this will contribute to reassuring the Japanese public.
Japanese officials said earlier the situation remains very serious but is relatively stable.
Japan has been battling to bring the situation under control since the plant was battered by the massive earthquake and tsunami that hit the northeast of the country last week.
There have been concerns that a major breakout of radioactive pollution from the plant could pose a serious health risk, and China and other nearby countries have stepped up monitoring of radiation levels.
Millions of people in the capital, Tokyo, remain indoors although prevailing winds are likely to carry contaminated smoke or steam away from the city and over the Pacific Ocean.
The United States government is planning to fly home thousands of American military families and civilians from Japan.
Plant operator Tokyo Electric Power Co says engineers are having to be constantly checked for radiation levels.
The head of the IAEA has headed to Japan from Vienna with a group of the agency's nuclear experts.
Yukiya Amano says the situation continues to be very serious.