The fleet said that the radiation was from a plume of smoke and steam released from the crippled Fukushima nuclear power plant.
The US Seventh Fleet has now moved its ships and aircraft away from the nuclear power plant after discovering low-level radioactive contamination.
The aircraft carrier USS Ronald Reagan was about 160 kilometres offshore when its instruments detected the radiation.
But the fleet says the dose of radiation was about the same as one month's normal exposure to natural background radiation in the environment.
There was no indication that any of the military personnel had experienced ill effects from the exposure, the New York Times reported.
But the episodes showed that the prevailing winds were picking up radioactive material from crippled reactors in northeastern Japan, The NY Times noted.
Japan has been battling to control the overheating No. 1 and No. 3 reactors at the ageing Fukushima plant after the cooling systems were knocked out by Friday's 9-magnitude quake and the resulting tsunami that swallowed towns.
A new explosion at the plant today had experts racing to avert a catastrophic meltdown in the wake of the quake and tsunami.