WASHINGTON - The United States showed increasing alarm about Japan's nuclear crisis on Wednesday and urged its citizens to stay clear of an earthquake-crippled power plant, going further in its warnings than Japan itself.

The State Department said the United States has chartered aircraft to help Americans leave Japan and had authorized the voluntary departure of family members of diplomatic staff in Tokyo, Nagoya and Yokohama -- about 600 people.

"The State Department strongly urges US citizens to defer travel to Japan at this time and those in Japan should consider departing," it said.

As operators of the Fukushima plant tried 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," White House spokesman Jay Carney told reporters.

US officials took pains not to criticize the Japanese government, which has shown signs of being overwhelmed by the crisis that began after last Friday's devastating 9.0 magnitude earthquake and resulting tsunami.

But Washington's actions indicated a divide with the Japanese about the perilousness of the situation.

US President Barack Obama told Japanese Prime Minister Naoto Kan in a telephone call that the United States will do all it can to help Japan recover, the White House said.

"The president briefed Prime Minister Kan on the additional support being provided by the US, including specialized military assets with expertise in nuclear response and consequence management," it said in a statement.

The State Department recommended that US citizens within 50 miles (80 km) of the plant leave the area or stay indoors.

Japan's government has asked people living within 12 miles (20 km) to evacuate and those between 12 miles and 18 miles (30 km) to stay indoors.

PLUME OF RADIATION

Gregory Jaczko, the top US nuclear regulator, cast doubt on efforts to cool overheating reactors, saying workers may be hit with "lethal doses" of radiation.

"It would be very difficult for emergency workers to get near the reactors," Jaczko said.

A United Nations forecast projects the radioactive plume from the Fukushima facility would reach the Aleutian Islands on Thursday and hit Southern California late on Friday, The New York Times reported.

The projection, calculated on Tuesday and obtained by the newspaper, gives no information about actual radiation levels, it said. Health and nuclear experts emphasize that radiation in the plume will be diluted as it travels and will have extremely minor health consequences in the United States, it reported.

The US military has ordered its forces to stay 50 miles (80 km) away from the plant, the Pentagon said. There are at least 55,000 members of the US forces in Japan and offshore assisting the relief operation.

"All of us are heartbroken by the images of what's happening in Japan and we're reminded of how American leadership is critical to our closest allies," Obama said in Washington.

"Even if those allies are themselves economically advanced and powerful, there are moments where they need our help, and we're bound together by a common humanity."

CONFLICTING REPORTS

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 ground level, US Energy Secretary Steven Chu told a congressional hearing.

The detection system is part of equipment and 39 personnel from the Energy Department sent to Japan, he said. It has also provided equipment to monitor airborne radiation.

The United States is deploying more 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.

"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 ... 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 Secretary of State Hillary Clinton said on Wednesday the 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," she said in an interview with MSNBC in which she emphasized the need for a comprehensive US energy policy.

"We get 20 percent of our energy right now in the United States from nuclear power."