SEOUL - US Vice President Joe Biden met with South Korean President Park Geun-Hye on Friday, on the final leg of a three-nation Asian tour dominated by regional security concerns.
China's declaration of a new air defence zone and North Korea's nuclear weapons programme was to be the main focus of Biden's meetings in Seoul, where he will also deliver a keynote policy speech at Yonsei University.
The threat posed by Pyongyang was underlined by the publication of new satellite images that appeared to show stepped up activity at North Korea's main nuclear site in line with the regime's vows to expand its weapons programme.
President Park was expected to press Biden Friday morning on the issue of China's new "air defence identification zone" (ADIZ) which has not only inflamed China's territorial disputes with Japan, but also overlaps South Korea's own ADIZ.
Seoul has threatened to announce the expansion of its ADIZ in retaliation - a move Biden is expected to discourage as Washington seeks to ratchet down tensions in the region.
During his stops in Tokyo and Beijing, Biden underlined Washington's opposition to Beijing's move, but stopped short of demanding that China rescind the air zone.
Tensions in the region are at their highest in years, with China and Japan squaring off over a chain of uninhabited islands in a feud that has some observers warning of the danger of an armed confrontation.
Seoul has also denounced the new Chinese air zone which covers a tiny, South Korean-controlled rock in the East China Sea that is also claimed by Beijing.
As well as seeking to reassure Park, Biden will be looking to encourage Seoul to pursue better relations with Tokyo.
As the battle for influence in Asia between China and the United States heats up, Washington wants its two main military allies in the region on board and undivided.
But South Korea and Japan are going through one of their regular diplomatic freezes, with Park refusing to even talk to Prime Minister Shinzo Abe until Tokyo apologises for a host of historical grievances related to Japan's 1910-45 rule over the Korean peninsula.
On Saturday, Biden is scheduled to tour the demilitarised zone that has separated South and North Korea since the end of the 1950-53 Korean War.
His visit comes days after South Korea's intelligence agency reported a major purge in the North Korean leadership, with the apparent ousting of leader Kim Jong-Un's uncle and political mentor Jang Song-Thaek.
The report triggered fevered speculation about the stability of the regime in Pyongyang at a time when North Korea is pressing ahead on all fronts with its nuclear weapons programme.
China has been pushing hard for Washington and Seoul to resume six-party talks on the North's nuclear ambitions, but they insist Pyongyang must first demonstrate tangible commitment to denuclearisation.
"The dialogue has to be designed as something other than just either a dead-end or talking for the sake of talking," a senior White House official told reporters during Biden's stop in China.
The United States is currently trying to secure the release of two of its citizens being held in North Korea, including an 85-year-old Korean War veteran.
In Beijing on Thursday, Biden warned China against escalating the dispute over its new ADIZ, arguing that regional peace and stability were in Beijing's interests.
"As China's economy grows, its stake in regional peace and stability will continue to grow as well, because it has so much more to lose," Biden said.
"That's why China will bear increasing responsibility to contribute positively to peace and security," he added.