In a statement at the White House, Obama noted that the Nuclear Regulatory Commission on March 16 called for US citizens to evacuate to beyond a 50-mile radius of the stricken plants in northeastern Japan.

That figure exceeds the 12.4-mile evacuation radius and 18.6-mile shelter radius called for by the Japanese government. Obama said the stricter US recommendation was "based upon a careful scientific evaluation."

He also noted that the government has authorised the voluntary departure of family and dependents of US officials in northeast Japan, and he called on all US citizens in Japan "to carefully monitor the situation."

Meanwhile, the United States is "working aggressively" to offer support, with search-and-rescue teams and disaster-response teams and members of the US military working in Japan.

"To date, we have flown hundreds of missions to support the recovery efforts and distributed thousands of pounds of food and water."

He added, "We're sharing with them expertise, equipment and technology so that the courageous responders on the team have the benefit of American teamwork and support."