The head of the UN's nuclear energy agency says Japan is racing against the clock to cool the overheating nuclear reactors at its crippled power plant.

International Atomic Energy Agency head Yukiya Amano gave the critical assessment after flying into Tokyo and being briefed by Japanese Prime Minister Naoto Kan. Amano says he underscored his concern about the extreme seriousness of the accident at the Fukushima Dai-ichi nuclear plant, the Associated Press reports.

A week after the earthquake and tsunami damaged the plant, authorities are still trying to reconnect a power supply to restart cooling systems. One reactor, Unit 3, is drawing urgent concern, with its nuclear fuel storage pool dangerously low of water, exposing the stored fuel rods.

Methods of cooling the reactors have included water bombing by helicopters and sending fire engines into action to douse the reactors and containment pools and stop fuel rods from degrading.

"We poured water onto the number-three reactor yesterday. There is no doubt that water entered the pool, but we have not confirmed how much water is in there," said chief government spokesman Yukio Edano.. "We will go ahead and continue to discharge water on the reactor this afternoon. The number-three reactor is the top priority."

The plant's operator Tokyo Electric Power Company (TEPCO) said overnight it believed the effort was bearing fruit, despite skepticism from some international experts who have viewed it as a desperate measure.