Barack Obama arrived in Seoul today to growing signs North Korea was defiantly readying another nuclear test, and with South Koreans in a state of national mourning over the sinking of a ferry full of schoolchildren.
The American president landed at a US Air Force base outside a South Korean capital still wracked with grief over the 300 people dead or missing more than a week after the disaster.
Obama is expected to offer personal condolences to his counterpart Park Geun-Hye over the tragedy, but the South's unpredictable northern neighbour is set to dominate the agenda.
Satellite photos taken just two days ago showed additional activity at North Korea's Punggye-ri test site that is "probably related to preparations for a detonation", the US-Korea Institute at Johns Hopkins University said.
The report echoed recent warnings from South Korea that the North might be planning a test to coincide with Obama's two-day visit.
If Pyongyang presses ahead with its fourth nuclear test it would be a clear challenge to Obama's bid to cement America's role as a Pacific power.
His four-nation Asian tour began in Tokyo, where he urged China to rein in its wayward ally, saying Beijing had a "critically important" role to play in defusing tensions on the volatile peninsula.
"North Korea has engaged in provocative actions for the last several decades," he said. "It's been an irresponsible actor on the international stage for the last several decades.
"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."
In an interview ahead of his arrival in Seoul, he warned North Korea could expect a "firm response" if it made "the mistake" of conducting another nuclear test.
Pyongyang, for its part, slammed Obama's trip earlier this week as a "dangerous" move that would escalate military tension and bring the "dark clouds of a nuclear arms race" over the Korean peninsula.
Adding to the tense mix was the news that a South Korean naval vessel had fired warning shots after two North Korean patrol boats crossed the disputed maritime border day. The boats quickly retreated.
