U.S. Secretary of State John Kerry was en route to Japan on Sunday, the last stop on an Asian tour aimed at reining North Korea's nuclear ambitions.
Kerry met China's top leaders on Saturday in a bid to persuade them to push reclusive North Korea, whose sole main ally is Beijing, to scale back its belligerence and return to nuclear talks after weeks of threats of nuclear attacks on the United States and South Korea.
In Seoul, Kerry said North Korea would be making a "huge mistake" if it launched one of its medium-range missiles during the current standoff. Japan, separated by less than 1,000 km (625 miles) of water and a frequent target of North Korea's anger, is in easy range.
Also likely to be high on the agenda in talks in Tokyo are Japan's territoral disputes with China and the future of U.S. bases in Japan.
The United States and Japan this month announced an agreement for the return to Japan of a U.S. air base, taking a step to resolving an issue that trouble relations.