TOKYO - The US Seventh Fleet said Monday it had moved its ships and aircraft away from a quake-stricken Japanese nuclear power plant after discovering low-level radioactive contamination.

The fleet said that the radiation was from a plume of smoke and steam released from the crippled Fukushima Dai-Ichi Nuclear Power Plant, where there have been two hydrogen explosions since Friday's devastating earthquake and tsunami.

The aircraft carrier USS Ronald Reagan was about 100 miles (160 kilometers) offshore when its instruments detected the radiation. The fleet said the dose of radiation was about the same as one month's normal exposure to natural background radiation in the environment.