The United States showed increasing alarm about the worsening nuclear situation in Japan on Wednesday and urged its citizens to stay clear of an earthquake-crippled power plant -- going further in its warnings than Japan itself.
As operators of the Fukushima plant pledged to try again to use helicopters to douse overheating reactors, US officials warned about the risks of getting anywhere near the area and relied on their own officials for details about the danger.
"The situation has deteriorated in the days since the tsunami and ... the situation has grown at times worse with potential greater damage and fallout from the reactor," said President Barack Obama's spokesman, Jay Carney.
US officials took pains not to criticize the Japanese government, which has shown signs of being overwhelmed by the crisis, but Washington's actions indicated a divide with the Japanese about the perilousness of the situation.
The State Department recommended that US citizens within 50 miles (80 km) of the plant leave the area or stay indoors.
Japan's government, meanwhile, has asked people living within 12 miles (20 km) to evacuate and those between 12 miles and 18 miles (30 km) to stay indoors.
The top US nuclear regulator cast doubt on emergency workers' ability to cool overheating reactors, saying radiation levels may give them "lethal doses" of radiation.
"We believe that around the reactor site there are high levels of radiation," said Gregory Jaczko. "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."
The US military has also ordered its forces to stay 50 miles (80 kilometers) away from the plant, the Pentagon said. There are at least 55,000 US forces in Japan and offshore assisting the relief operation.
"We are concerned enough that we have offered a great deal of assistance to the Japanese and we have our own experts on the ground both assisting and evaluating information independently," Carney said.
High radiation levels prevented a helicopter from dropping water into the stricken Japanese plant's No. 3 reactor to cool its fuel rods after an explosion damaged the roof and cooling system. Operators planned to try again on Thursday.
The State Department's warning to US citizens was based on new information collected by the US Nuclear Regulatory Commission, Department of Energy and other US sources.
The United States is trying to deploy equipment in Japan that can detect radiation exposure at the ground level, US Energy Secretary Steven Chu told a congressional hearing.
The detection system is part of the 17,200 pounds (7,800 kg) of equipment and 39 personnel from the Energy Department sent to Japan, he said. The department has also provided equipment to monitor airborne radiation.
The United States is deploying additional radiation monitors on Hawaii and other US islands even though it does not expect harmful levels of radiation to reach US soil, environmental regulators said.
Chu declined to tell lawmakers, when asked, whether he was satisfied with Japan's response so far to its nuclear crisis, which began after last Friday's devastating 9.0 magnitude earthquake and resulting tsunami.
"I can't really say. I think we hear conflicting reports," Chu said.
"This is one of the reasons why (the United States is) there with boots on the ground, with detectors in the ground, not only to help assist (the) Japanese power company and the Japanese government but also for our own sake -- to know what is really happening."
Beyond the risk to workers at or near the damaged nuclear plant, one scientist, Dr Ira Helfand, warned of possible widespread contamination of people and land.
"We need ... to focus on the radioactive isotopes being dispersed at some distance from the plant, because this is going to cause a whole different set of health problems," Helfand, past president of the anti-nuclear group Physicians for Social Responsibility, said in a telephone briefing.
The Obama administration has maintained its support for expanding US use of nuclear energy despite renewed fears about its safety after the events in Japan.
But US Secretary of State Hillary Clinton said on Wednesday that the nuclear crisis raised questions about the use of nuclear energy in the United States.
"What's happening in Japan raises questions about the costs and the risks associated with nuclear power, but we have to answer those. We get 20% of our energy right now in the United States from nuclear power," she said in an interview with MSNBC in which she emphasized the need for a comprehensive US energy policy.