The U.S. Embassy in Japan has asked American citizens living within an 80-kilometer radius of the quake-hit Fukushima No. 1 nuclear plant in Japan to evacuate as a precautionary measure.

‘‘We are recommending, as a precaution, that American citizens who live within 50 miles (80 kilometers) of the Fukushima Nuclear Power Plant evacuate the area or to take shelter indoors if safe evacuation is not practical,’’ the embassy said in the advice issued Thursday.

Conditions such as weather and wind direction will affect the area of radioactive contamination in a complex way, the embassy said, adding that low-level radioactive materials can reach areas more than 80 km away from the damaged nuclear power plant.

The Japanese government currently sets the evacuation zone covering areas within a 20-km radius of the plant and advises those within a 30-km radius to stay indoors.

Britain on Wednesday advised its nationals living in Tokyo and areas north of the capital to evacuate. It said the British Embassy plans to arrange free buses from Sendai, Miyagi Prefecture to Tokyo on Thursday for evacuating British nationals. It also advised against all non-essential travel to Tokyo and northeastern Japan.

‘‘For those outside the exclusion zone set up by the Japanese authorities there is no real human health issue that people should be concerned about,’’ the Foreign Ministry said in a statement.

‘‘However, due to the evolving situation at the Fukushima nuclear facility and potential disruptions to the supply of goods, transport, communications, power and other infrastructure, British nationals currently in Tokyo and to the north of Tokyo should consider leaving the area,’’ it said.