Barack Obama arrived in Seoul Friday 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 Friday. The boats quickly retreated.
Spy satellite images of test site
North Korea watchers have puzzled over whether the test preparations they have seen via spy satellites are real, or bravado aimed at stealing the limelight during the US president's tour.
But the latest images suggested increased movement of vehicles and materials near what are believed to be the entrances to two completed test tunnels, the US-Korea institute said on its closely followed 38 North website.
Also visible were probable command and control vehicles intended to provide secure communications between the test site and other facilities.
North Korea has conducted three nuclear tests in 2006, 2009 and 2013.
The 38 North analysis noted that preparations for the test in February last year had peaked two or three days before detonation.
Any nuclear test would overwhelm the narrative of Obama's regional tour, billed as a chance to reinvigorate his rebalancing of US strategy towards the region.
But it would also have the effect of uniting Japan, South Korea and the United States in condemnation and would place China in a deeply embarrassing spot.
While a US presidential visit would normally be expected to command the lion's share of attention in South Korea, the country remains preoccupied with the misery wrought by the sinking of the passenger ferry.
Divers were Friday racing against time and tide to recover the 121 bodies still believed trapped in the sunken vessel, with bad weather expected to close in on Saturday, hampering the effort.
In an interview with the JoongAng Ilbo ahead of his visit, Obama said he and his wife, Michelle, offered their "deepest condolences to the South Korean people".
"As parents we cannot begin to imagine what all those grieving parents are going through having lost their sons and daughters."
After a summit and a formal dinner with Park later in the day, Obama on Saturday will visit some of the 28,500 US troops stationed in South Korea, before heading on to Malaysia and the Philippines.