US President Barack Obama has arrived in South Korea amid heightened tensions in the region over North Korea's hectic activity at its nuclear test site.
Obama, who is on the second leg of his Asia tour, will hold talks with South Korean President Park Geun-hye. The talks are likely to be dominated by the tensions in the Korean peninsula.
The US leader is also expected to express grief over the ferry tragedy in South Korea in which at least 300 people are either dead or missing.
This is Obama's fourth visit to Seoul, making the Asian country his most frequent destination while on presidential duty.
The American president, who flew in from Japan, will proceed to Malaysia and the Philippines after his two-day visit to South Korea.
In Japan, Obama said Pyongyang's revival of activity, widely interpreted as North Korea's preparation for its fourth nuclear test, at its test site Punggye-ri is "the most destabilising and dangerous" situation currently afflicting the Asia-Pacific region.
"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. 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" Obama said.
He added that China's role in the geopolitical game involving North Korea is "critically important".
After bilateral discussions between the US and South Korea, the two leaders are expected to hold a press conference voicing strong criticism against North Korea over its nuclear activities.
The reclusive nation carried out three nuclear tests in 2006, 2009 and 2013, prompting strong international condemnation.
According to the latest report released by the US-Korea Institute at John Hopkins University's 38 North, which specialises in North Korean affairs, Pyongyang's increased activity at its nuclear test site is "probably related to preparations for a detonation".