Top US officials appear to be pressing China to do more to rein in North Korean aggression. Meanwhile, the South held a major civil defense drill Wednesday.
China's highest-ranking foreign affairs official went to Pyongyang on Thursday, just after US Adm. Mike Mullen visited Japan and South Korea.
As a growing nuclear power plant builder and exporter, South Korea wants to reprocess nuclear fuel. That's counter to a US deal.
Obama is expected to receive a warm welcome in South Korea, where top issues on agenda are North Korea talks and a free trade agreement.