US President Barack Obama speaks at a DNC event in Washington, March 16, 2011. [Photo/Agencies]

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 of the plant leave the area or stay indoors.

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