Obama faces emotional trip to South Korea

US President Barack Obama will encounter a nation mourning one of its worst maritime disasters and on edge over North Korea's nuclear brinkmanship when he flies to South Korea.

The trip to Seoul was once billed as a routine, informal stop to catch up with old friends, but took on poignant overtones after the sinking of a ferry packed with high school students last week.

US intelligence agencies meanwhile are watching and waiting amid indications that North Korea could rattle the peninsula with a fourth nuclear test, in a clear challenge to Obama's strategy of cementing America's role as a Pacific power.

Obama will fly on Friday to Seoul from Tokyo, where he warned on the first stop of an Asian tour that China must play a "critically important" role in reining in its nominal ally in Pyongyang.

"North Korea has engaged in provocative actions for the last several decades.

"It's been an irresponsible actor on the international stage for the last several decades," he said.

"They are the most isolated country in the world. They are subject to more international sanctions and international condemnation than any country in the world."

With South Korea still preoccupied by scenes of misery and grief after the sinking of the Sewol last week, Obama will be expected to offer words of comfort and condolence when he meets President Park Geun-Hye.

The president has some idea what Park is going through as recriminations fly over the disaster, after serving as mourner-in-chief following several US tragedies, including the Newtown school shooting and the Boston bombing.

In a show of respect for the key US ally, Obama paused before a news conference devoted to domestic politics and the Ukraine crisis last week to empathise with the agony of South Koreans who had lost their children.

After a working dinner with Park on Friday, Obama will visit US troops, before heading on to Malaysia, on the third leg of his Asian tour, which also includes the Philippines.