Barack Obama warned North Korea today that the United States "will not hesitate to use our military might" to defend its allies.
The US president made a speech in Seoul, South Korea, showcasing America's power in the region amid China's growing influence and Pyongyang's unpredictable nuclear threats.
Mr Obama's visit came as North Korea threatened to conduct its fourth nuclear test, leading the president to raise the possibility of further sanctions.
"The commitment that the United States of America has made to the security of the Republic of Korea only grows stronger in the face of aggression," Mr Obama told some of the 28,000 American service members stationed in South Korea to keep watch on its northern neighbour.
"Our alliance does not waiver with each bout of their attention seeking. It just gains the support of the rest of the world."
The website 38 North, which closely monitors North Korea, said commercial satellite imagery from Wednesday showed increased movement of vehicles and materials near what are believed to be the entrances to two completed tunnels at the Punggye-ri nuclear test site.
The movements could be preparations for an underground atomic explosion, although predicting underground tests is notoriously difficult.
Mr Obama ridiculed North Korea's attempt to show force, saying: "Anybody can make threats. Anyone can move an army. Anyone can show off a missile. That doesn't make you strong."
He said real strength came from having an open participatory democracy, open markets and a society free to speak out against its government.
"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," Mr Obama said to cheers from the uniformed troops at a field house at Yongsan Garrison, the headquarters for US forces in South Korea.
The president's 10-minute speech followed a rare joint defence briefing with South Korean president Park Geun-hye that focused on efforts to counter the North's nuclear ambitions.
US Army General Curtis Scaparrotti, commander of the joint US-South Korea command, told the two presidents that his team "works together every day to make sure that we defend the Republic of Korea and that we deter North Korea".
Mr Obama then headed for Malaysia, the third stop on his four-country Asia visit. The mission of the trip is to underscore US commitment to the region at a period of uncertainty between North Korea's provocations and China's growing power.
While the US has long been the most powerful military influence in the Asia-Pacific region, Pentagon spending is being slashed at the same time China has been boosting its defence budget.
Beijing still lags far behind the US in both military funding and technology. But its spending boom is attracting new scrutiny at a time of severe cuts in America's defence budgets that have some questioning Washington's commitments to its Asian allies, including some who have lingering disputes with China.
At the same time, the US military is seeking to redirect resources to the Asia-Pacific as it draws down its commitment in Afghanistan, though there is concern that budget cuts could threaten plans to base 60% of US naval assets in the region.
Chief of naval operations Admiral Jonathan Greenert recently warned that US capabilities to project power "would not stay ahead" of potential adversaries, given the fiscal restraints.
Earlier, Mr Obama promoted trade between the US and South Korea with executives from businesses including Hyundai, Samsung, Korean Air, Microsoft, Boeing, Goldman Sachs and others.
"As important as the security relationship is and the alliance is between the Republic of Korea and the United States, what is also important is the incredible and growing economic ties that are creating jobs and opportunity in both countries," he said.
Mr Obama also paid tribute to victims from last week's ferry disaster.