US President Barack Obama has arrived in Seoul for talks with his South Korea counterpart Park Geun-hye.  But the long-scheduled visit is overshadowed by the ferry disaster off the south coast, and the threat of another rogue nuclear test far across the border into North Korea.

Mr. Obama will nudge Park to attempt to improve ties with Japan.  America needs its two main Asian allies South Korea and Japan to work together to tackle North Korea’s nuclear ambitions and provide a stable counter to an ascending China. 

But Seoul and Tokyo have disagreements over some tiny, disputed inlands in between the two countries, and unresolved historical tensions have severely strained the relationship.  Last month, Obama managed to get Park and Japanese Prime Minister Shinzo Abe to meet on the sidelines of a nuclear security summit in the Netherlands.

Obama will not have time to visit the scene of the ferry disaster, but that’s probably a good thing considering how disruptive it would have been to secure the area at a time when crews should be concentrating on the recovery.  The US President will pay respect to the victims from Seoul.

Earlier in Japan, Obama said the North Korean problem is “the most destabilizing, dangerous situation in all of the Asia-Pacific region”, and described China's role in influencing Pyongyang as “critically important”.

Earlier this week, South Korea's military said it had detected “a lot of activity” at the North's nuclear test site, suggesting a possible imminent nuclear test.  And the respected 38 North blog also confirmed increased activity at the site “probably related to preparations for a detonation”, based on satellite imagery.