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

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.