Fukushima nuclear complex is seen burning after an explosion on March 14th, the second of three following Friday's earthquake and tsunami.

US officials challenged the Japanese government's assessment of the radiation risks posed by the damaged nuclear complex, telling US civilians and military personnel to stay at least 50 miles from the facility, according to the Wall Street Journal on Thursday.

White House spokesman Jay Carney told reporters that chairman of U.S. Nuclear Regulatory Commission, Greg Jaczko, came to the White House to brief President Barack Obama on the "deteriorating situation" at the nuclear power plant in Japan, with the attendance of senior officials, including National Security Advisor Tom Donilon, Homeland Security Advisor John Brennan.

Carney said, during the briefing, Jaczko informed Obama that it was his recommendation that American citizens in Japan who are within a 50-mile (80-kilometer) radius of the reactors evacuate from that area.

On Wednesday, the Defense Department?prohibited?U.S. military personnel who are helping the relief efforts in Japan from entering areas within 50 miles of the country's quake and tsunami damaged Fukushima nuclear power plant.

Pentagon spokesman Dave Lapan said U.S. forces operating near the quake zone have been given orders that ban them from going within 50 miles of the plant without approval. He said there are currently 17,000 sailors and Marines afloat on 14 vessels in waters off Japan, and no one has yet been given approval.

As a precautionary measure, the Navy has begun giving potassium iodide pills to helicopter crews who could fly relief supplies or other missions into at risk areas.

The U.S. military is mainly providing assistance in transport and relief supply missions to the Japanese authorities, and it has sent high-pressure water pumps to the Fukushima power plant. Bad weather condition reportedly hampered some relief flights on Wednesday.