The chairman of the United States Nuclear Regulatory Commission gave a significantly bleaker appraisal of threat posed by the Japanese nuclear crisis than the Japanese government, saying on Wednesday that the damage at one crippled reactor was much more serious than Japanese officials had acknowledged and advising to Americans to evacuate a wider area around the plant than ordered by the Japanese government.

Gregory Jaczko, the chairman of the commission, said in Congressional testimony that the commission believed that all the water in the spent fuel pool at the No. 4 reactor of the Fukushima Daiichi Nuclear Power Station had boiled dry, leaving fuel rods stored there completely exposed. As a result, he said, “We believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures.”