President Barack Obama said on Saturday the United States did not use its military might to "impose things" on others, but that it would use that might if necessary to defend South Korea from any attack by the reclusive North.
The North warned last month it would not rule out a "new form" of atomic test after the U.N. Security Council condemned Pyongyang's launch of a mid-range ballistic missile into the sea east of the Korean peninsula.
Obama and South Korean President Park Geun-hye presented a united front against North Korea at a joint news conference following their summit on Friday, warning that they would respond firmly to any "provocations" by Pyongyang which routinely threatens the United States and South Korea with destruction.
"We don't use our military might to impose these things on others, but we will not hesitate to use our military might to defend our allies and our way of life," Obama told cheering U.S. forces at the Yongsan garrison on a sunny spring morning.
"So like all nations on Earth, North Korea and its people have a choice. They can choose to continue down a lonely road of isolation, or they can choose to join the rest of the world and seek a future of greater opportunity, and greater security, and greater respect - a future that already exists for the citizens on the southern end of the Korean peninsula."
North Korea is already subject to U.N. sanctions over its previous three atomic tests.
Recent satellite data shows continued work at the nuclear test site in North Korea, although experts analysing the data say that preparations do not appear to have progressed far enough for an imminent test.
Adding to tensions surrounding Obama's visit to South Korea, the North announced on Friday it had detained a 24-year-old American this month who demanded asylum after arriving in the country on a tourist visa.
Obama is using his week-long Asia tour to try to ease doubts among U.S. allies about his promise to "rebalance" military, diplomatic and economic resources toward the fast-growing Asia-Pacific region.
He has sought to strike a balance between showing the United States will be a counterweight to China without alienating Beijing, which worries that Washington wants to contain its growth and influence.
Obama and Park also urged China, North Korea's main ally, to uses itsinfluence to help rein in its unpredictable neighbor.
Underscoring the vast differences between the economically dynamic South and the impoverished North, Obama met earlier with a business roundtable in Seoul where he hailed the benefits of a U.S.-South Korea trade agreement that took effect in 2012.
Obama's visit came at a time when South Koreans remain preoccupied with the aftermath of the sinking of a ferry carrying hundreds of youngsters, one of the worst tragedies to hit the country in modern times.
Stressing a deep U.S. bond with South Korea during his visit, Obama has expressed condolences to Park and the Korean people. More than 300 people drowned or are missing and presumed dead after the April 16 sinking. Investigations are focused on human error and mechanical failure.
Before visiting Seoul, Obama spent three days in Tokyo - the first full state visit by a U.S. president since 1996 - on a visit meant to show that the U.S.-Japan alliance, the main pillar of America's security strategy in Asia, is solid at a time of rising tensions over growing Chinese assertiveness and the North Korean nuclear threats.
He is also visiting Malaysia and the Philippines.