US President Barack Obama is in Seoul for a visit that comes amid concern that North Korea may be planning a fourth nuclear test.
Mr 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.
Mr 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.
Obama’s Asia tour
- 23 Apr: Arrives Tokyo (evening) for dinner with Japanese PM Shinzo Abe
- 24 Apr: In Tokyo; talks and joint press conference with Abe, state dinner
- 25 Apr: Flies Tokyo-Seoul; talks and press conference with South Korean President Park Geun-hye
- 26 Apr: Visit to military base; flies to Kuala Lumpur and state dinner
- 27 Apr: Talks with Malaysian PM Najib Razak, press conference
- 28 Apr: Flies to Manila, talks with Philippine President Benigno Aquino
- 29 Apr: Ends visit to Philippines, returns to US
Most of those who died were teenagers on a school trip.
‘Two test tunnels’Mr Obama is expected to hold talks with Ms Park, visit US troops and then fly to Malaysia on Saturday.
Speaking in Japan, he called the North Korean problem “the most destabilising, 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 behaviour, because of their trade ties.
North Korean nuclear tests
- Three underground nuclear tests have been carried out by North Korea, in 2006, 2009 and 2013
- The first two tests were believed to have used plutonium, but it is not clear whether or not the third test used uranium as the fissile material
- North Korea is thought to have enough nuclear material for a small number of bombs, but not the technology to make a nuclear warhead
- Multiple rounds of multi-national talks have failed to convince Pyongyang categorically to commit to giving up its nuclear ambitions
“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.”
‘Inherent territory’Also on Mr 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 Ms Park and Japanese Prime Minister Shinzo Abe in a bid to put things back on track.
In Japan, Mr Obama issued a firm statement of support over Tokyo’s dispute over a separate set of islands with China.
Standing next to the Japanese leader, he confirmed that the US-Japan security treaty did cover the disputed East China Sea island chain – meaning that if Japan were to be attacked, the US would come to its aid.
China said it opposed the US stance.
“No matter what other people say and do, they can’t change the fundamental fact that the Diaoyu islands (called Senkaku in Japan) are part of China’s inherent territory,” said spokesman Qin Gang.
