US Secretary of State Hillary Clinton is in Japan on a visit to show solidarity five weeks after the devastating earthquake and tsunami.
She is meeting Prime Minister Naoto Kan, who has had to contend with both a nuclear and a humanitarian crisis as a result of the disaster.
Mrs Clinton will also attend a tea party hosted by the Japanese Emperor and Empress.
The United States has won Japanese admiration for lending its navy to help in relief efforts and offering nuclear expertise.
Before the quake, ties had been strained by a dispute over US military bases.
US assistance to Japan has included the dispatch of nuclear experts to the crippled Fukushima nuclear plant.
It has supplied fire engines, pumps and radiation protective suits, as well as barges to carry fresh water to cool the reactors.
As part of its support, the US has mobilised more than 20,000 personnel, about 160 aircraft and 20 ships.
Meanwhile, the operator of the Fukushima nuclear plant has said it aims to reduce radiation leaks within three months and to achieve a ‘cold shutdown’ within six to nine months.
Japan's embattled Tokyo Electric Power Company offered the timeline more than five weeks after the giant quake and tsunami knocked out cooling systems at its six-reactor Fukushima Daiichi atomic power station.
Radiation has since leaked into the air, soil and sea from the coastal plant northeast of Tokyo, as emergency crews have doused overheating reactors and fuel rod pools to prevent full meltdowns of volatile fuel rods.
