President Barack Obama arrived in Seoul for a visit that comes amid concern that North Korea may be planning a fourth nuclear test.
Barack Obama, who arrived from Japan on the second stop of his Asian tour, will hold talks with South Korean leader Park Geun-hye.
Their talks are set to focus on North Korea, following reports of activity at the Punggye-ri nuclear test site.
Barack Obama is also expected to express grief over last week’s ferry disaster.
More than 300 people were killed or remain missing after the Sewol passenger ferry sank off South Korea, in a tragedy that has shocked the nation.
Most of those who died were teenagers on a school trip.
Barack Obama is expected to hold talks with Park Geun-hye, visit US troops and then fly to Malaysia on Saturday.
Speaking in Japan, the US president called the North Korean problem “the most destabilizing, dangerous situation in all of the Asia-Pacific region”, and described China’s role in influencing Pyongyang as “critically important”.
Earlier this week, South Korea’s military said it had detected “a lot of activity” at the North’s nuclear test site, suggesting it was either planning a test or would pretend to stage one.
North Korea has carried out three such tests in the past, most recently in February 2013 – an incident that triggered months of severe tension on the Korean peninsula.
It also carried out tests in 2006 and 2009. All resulted in the imposition of sanctions by the UN, which bars Pyongyang from nuclear tests under resolution 1718.
A report from 38 North, the website of the US-Korea Institute at Johns Hopkins University, also confirmed increased activity at the site “probably related to preparations for a detonation”, based on satellite imagery.
The document highlighted “increased movement of vehicles and materials near what are believed to be the entrances to two completed test tunnels”.
While Pyongyang has tested devices, it is not yet believed to have mastered the process of making a nuclear warhead small enough to deliver via a missile.
China is regarded as the nation with the best chance of influencing North Korea’s behavior, because of their trade ties.
“We will not allow war and chaos on China’s doorstep,” foreign ministry spokesman Qin Gang said on Thursday.
“In the meantime, we have consistently and proactively advocated dialogue and negotiation.”
Also on Barack Obama’s agenda will be Seoul’s ties with Japan. The US wants its two main Asian allies to work together to tackle North Korea’s nuclear ambitions.
But rows over disputed islands and unresolved historical tensions have severely strained the Tokyo-Seoul relationship.
Last month, the US brokered a meeting between South Korea’s President Park Geun-hye and Japan’s PM Shinzo Abe in a bid to put things back on track.
In Japan, Barack Obama issued a firm statement of support over Tokyo’s dispute over a separate set of islands with China.
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