US President Barack Obama will encounter a nation mourning one of its worst maritime disasters and on edge over North Korea's nuclear brinkmanship Friday 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 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."

US officials were puzzling over the always unpredictable Kim's next move -- wondering whether he would show his defiance with a blast during Obama's Asian tour or if activity at the North's nuclear sites was a mere propaganda move.

Certainly, any nuclear test would overwhelm the narrative of Obama's regional tour designed to reinvigorate his rebalancing of US strategy towards the region.

But it would also have the affect of uniting Japan, South Korea and the United States in condemnation of the test and would place China in a deeply embarrassing spot.

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.

"Our hearts ache to see our Korean friends going through such a terrible loss, especially the loss of so many young students," he said.

The confirmed death toll stood at 175, but 127 were still unaccounted for, their bodies believed trapped in the submerged ship that sank a week ago, according to Yonhap news agency.

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.