The cause of the South Korean ship sinking is unclear. But North Korea has taunted the South in the past, and the two navies exchanged gunfire last year.
As China rises and the dollar falls, the US ally seeks more independence, but not less security, from America. Tokyo is the first stop on the president's Asia tour.
North Korea said Tuesday that it has reprocessed 8,000 nuclear fuel rods – perhaps enough to build another nuclear bomb. US officials say this was a violation of North Korea's commitments to nuclear disarmament talks.
Other countries, too, are part of the effort to block the transport of nuclear materials.
Defense Secretary Gates, off to Singapore for a regional security summit, says US sees no 'crisis' in Pyongyang's 'very provocative' display of force this week.