TOKYO (Reuters) - Japanese military helicopters dumped water on an overheating nuclear plant on Thursday while the United States expressed growing alarm about leaking radiation and said it was sending aircraft to help Americans leave the country. Engineers tried to run power from the main grid to start water pumps needed to cool two reactors and spent fuel rods considered to pose the biggest risk of spewing radioactivity into the atmosphere as the capital faced possible widespread powercuts. Emergency crews temporarily withdrew a water cannon from Japans quake-stricken nuclear power plant on Thursday because of high radiation levels, broadcaster NHK said. The water cannon had been called in to spray the Fukushima Daiichi complexs No.3 reactor, which contains plutonium fuel and has been the top priority for authorities. While Japanese officials scrambled with a patchwork of fixes, the top US nuclear regulator warned that the cooling pool for spent fuel rods at reactor No 4 may have run dry and another was leaking. Gregory Jaczko, head of the Nuclear Regulatory Commission, told a parliamentary hearing that radiation levels around the cooling pool were extremely high, posing deadly risks for workers still toiling in the wreckage of the earthquake-shattered 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, he said in Washington. Japans nuclear agency said it could not confirm if water was covering the fuel rods. The plant operator said it believed the reactor spent-fuel pool still had water as of Wednesday, and made clear its priority was the spent-fuel pool at the No 3 reactor. On Thursday morning alone, military helicopters dumped around 30 tonnes of water, all aimed at this reactor. Inside the complex, torn apart by four explosions since a 9.0-magnitude earthquake and tsunami hit last Friday, workers in protective suits and using makeshift lighting tried to monitor what was going on inside the six reactors. They have been working in short shifts to minimise radiation exposure. US officials took pains not to criticise the Japanese government, which has shown signs of being overwhelmed by the crisis, but Washingtons actions indicated a divide with its close ally about the perilousness of the situation. The worst-case scenario doesnt bear mentioning and the best-case scenario keeps getting worse, Perpetual Investments said in a note on the crisis. Japan said the United States would fly a high-altitude drone over the stricken complex to gauge the situation, and had offered to send nuclear experts.