Barack Obama, the US president, has landed at a US Air Force base outside Seoul, South Korea's capital, amid growing signs that its northern neighbour is preparing for a nuclear test.
Obama offered personal condolences to his counterpart Park Geun-Hye over the country's recent ferry tragedy, but North Korea was expected to dominate the agenda.
Satellite photos taken 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, cited by the AFP news agency, echoed recent warnings from South Korea that the North might be planning a test to coincide with Obama's two-day visit.
Al Jazeera's Harry Fawcet, reporting from Seoul, said indications that North Korea was preparing for a nuclear test had come out regularly in the past couple of weeks.
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 the South Korean defence ministry had been monitoring increased activity at the Punggye-ri nuclear test site, adding that a "detonation could perhaps be imminent".
If Pyongyang presses ahead with its fourth nuclear test it would be a clear challenge to Obama's bid to cement Washington's role as a Pacific power.
Obama offered South Korea heartfelt condolences for its "incredible loss" as the country struggles to come to terms with the sinking of a vessel packed with schoolchildren.
He also said it may be time to consider further sanctions against North Korea "that have even more bite".
The two leaders bowed their heads in a moment of silence, and Obama handed Park the US flag which had flown over the White House on the day the ferry sank on April 16 as a gesture of respect from the American people.
"I'm very mindful that my visit comes at a time of mourning for the people of this nation," Obama said before talks with Park at the presidential Blue House.
The president's four-nation Asian tour began in Tokyo, where he urged China to rein in North Korea, 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, Obama 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 on Friday. The boats quickly retreated.
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.