The US vice president has said there should be no doubt about US commitment to its strategic shift to Asia.
Speaking on Friday in Seoul, the South Korean capital, Joe Biden said there would be no change to President Barack Obama's new security strategy that emphasises a shift, or "pivot", towards the Asia region in recognition of China's growing military power.
In talks with South Korean President Park Geun-Hye, and later in a speech at Seoul's Yonsei University, Biden reiterated US opposition to a new Chinese air-defence identification zone that has stirred regional tensions - especially between China and Japan.
For her part, Park pressed Biden on China's new "air-defence identification zone" (ADIZ) which, as well as inflaming China's territorial disputes with Japan, also overlaps South Korea's own ADIZ.
South Korea has threatened to announce the expansion of its ADIZ in retaliation - a move Biden was seeking to discourage as the US seeks to calm what is already a dangerously volatile mood in the region.
Park said Biden's trip to the region would be "of much help for peace" in Northeast Asia.
"We are at a point in time when the situation in Northeast Asia is very fluid and tensions in the region are escalating," she said.
Acknowledging the "considerable apprehension" caused by China's declaration, Biden stressed that the US did not recognise the new zone.
"America is a Pacific power - a resident Pacific power - and we are going nowhere. Nowhere," he said.
Pivot to Asia reaffirmed
As the two leaders sat down for talks, Biden said: "I want to make one thing absolutely clear: President Obama's decision to rebalance to the Pacific basin is not in question.
"The United States never says anything it does not do. It's never been a good bet to bet against America ... and America will continue to place its bet on South Korea."
He also underlined the regional and global unity in the face of the "clear and present danger" of North Korea's nuclear weapons programme.
"Let there be no doubt, the United States is committed to do what it takes to defend our allies and ourselves against North Korean aggression. Period," Biden said.
He is scheduled to tour on Saturday the demilitarised zone that has separated South and North Korea since the end of the 1950-53 Korean War.
China has been pushing hard for the US and South Korea to resume six-party talks on the North's nuclear ambitions, but Biden insisted North Korea must first demonstrate tangible commitment to denuclearisation.
His visit comes just days after South Korea's intelligence agency reported a major purge in the North Korean leadership, with the apparent toppling of leader Kim Jong-un's uncle and political mentor Jang Song-thaek.
The report prompted intense 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.
New satellite images
The threat posed by North Korea was underlined by the publication on Thursday of new satellite images that appeared to show increased activity at North Korea's main nuclear site, in line with the regime's vows to expand its weapons programme.
Seoul was Biden's last stop on a three-country Asia tour that has already taken him to Japan and China.
However, as on his visits in Tokyo and Beijing, he stopped short of demanding that China rescind its decision.
While reassuring South Korea of US support, Biden's university speech clearly underscored US desire for the country to pursue better relations with Japan.
As the battle for influence in Asia between China and the US heats up, the US 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 Shinzo Abe, the Japanese PM, until his country apologises for a host of historical grievances related to Japan's 1910-45 rule over the Korean peninsula.