The race is on to maintain cooling of used nuclear fuel at the damaged Fukushima Daiichi power plant in Japan.
Japanese military helicopters dumped water on the plant while engineers tried to power up the water pumps needed to cool two reactors and spent fuel rods which are at risk of releasing radioactivity into the atmosphere.
The cooling ponds are the priority for authorities as they contain highly radioactive heat generating nuclear fuel and require an adequate level of water to be maintained as well as pumped circulation to control water temperature.
Normal operating levels are at 25 degrees Celsius, and at higher temperatures cooling by natural convection begins to be markedly less effective.
The US Nuclear Regulatory Commission has warned that the cooling pool for spent fuel rods at reactor No.4 may have run dry and another was leaking.
High levels of radiation and presence of hydrogen at the unit indicate that fuel is uncovered and suffering damage in the pond, posing a deadly risk for engineers working at the power plant.
"It would be very difficult for emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time," said regulator head Gregory Jaczko.
Workers are trying to monitor the six reactors, wearing protective suits and working in short shifts to minimise radiation exposure.
Japan's nuclear agency said it could not confirm if water was covering the fuel rods, while the plant operator said it believed the reactor spent-fuel pool still had water and that its priority was the spent-fuel pool at the No.3 reactor - the only reactor that uses plutonium in its fuel mix.
The US plans to fly a high-altitude drone over the complex to gauge the situation, while Japan prepares eleven high pressure fire trucks to douse the units.
However heavy debris needs to be removed from the ground first, caused by explosions at the plant after the earthquake.
Officials plan to secure adequate water using helicopters and fire trucks in the short term to allow time for the restoration of existing piping systems for maintenance of water levels.
Sebastian Pflugbeil, president of the private German-based Society for Radiation Protection, said Japan's efforts to pull the Fukushima plant back from the brink signalled "the beginning of the catastrophic phase".
He added that a wind blowing any nuclear fallout east into the Pacific would limit any damage for Japan's 127 million people in case of a meltdown or other releases, for instance from spent fuel storage pools.