SEOUL (AFP) - Vice President Joe Biden said on Friday there should be no doubt about America's commitment to its strategic shift to Asia as he wound up a regional tour dominated by security concerns including a new Chinese air zone.
In talks with South Korean President Park Geun-Hye and later in a speech at Seoul's Yonsei University, Biden reiterated US opposition to the Chinese zone which has fuelled regional tensions - especially between Beijing and Tokyo.
At the same time, he 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," he said in his speech.
The threat posed by Pyongyang 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.
In his talks with Park, Biden stressed there would be no change to President Barack Obama's security strategy with its emphasis on a "pivot" towards Asia in recognition of China's growing military power.
"I want to make one thing absolutely clear: President Obama's decision to rebalance to the Pacific basin is not in question," Biden said at the start of the Park meeting.
"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 added.
"America is a Pacific power - a resident Pacific power - and we are going nowhere. Nowhere."
Seoul was Biden's last stop on a three-country Asia tour that has already taken him to Japan and China.
President Park pressed Biden on China's new "air defence identification zone" (ADIZ) which, as well as inflaming Beijing's territorial disputes with Japan, also overlaps South Korea's own ADIZ.
Seoul has threatened to expand its ADIZ in retaliation.
The United States is consulting South Korea about that, as it 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" triggered by China's declaration, Biden stressed anew that Washington did not recognise the new zone.
However, as on his visits in Tokyo and Beijing, he stopped short of demanding that China rescind its decision.
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.
While reassuring South Korea of US support, Biden's university speech clearly referenced Washington's desire for 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.
"The entire region will be more stable and more secure if the leading democracies - Japan, South Korea and the United States - are able to improve their relations and cooperation with one another," he said.
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 just 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.
China has been pushing hard for Washington and Seoul to resume six-party talks on the North's nuclear ambitions, but Biden insisted Pyongyang must first demonstrate tangible commitment to denuclearisation.
"The international community will not accept or tolerate a nuclear-armed North Korea," the vice president said.