In a direct challenge to North Korean leaders, President Barack Obama implored them “to have the courage to pursue peace” while warning of the wrath of the world if they do not.
Failure, he said, would mean a future without dignity, respect or hope for its people.
Mr Obama stood by his pledge for a globe without nuclear weapons, declaring flatly that the United States has more than it needs and can cut its arsenal without weakening its security or that of its allies.
That assessment put him on a collision course with congressional Republicans who say any significant cuts would undermine the US ability to deter aggression.
In unusually personal terms, Mr Obama said he spoke of his wish for further nuclear reductions as the president of the only nation ever to used nuclear weapons, as a commander whose country’s nuclear codes are never far from his side, and as a protective father eager to erase the threat of nuclear annihilation.
Mr Obama and other world leaders are in Seoul for a major international nuclear security summit.
Mr Obama plans to meet on the sidelines of the summit with several heads of government, including Russian and Chinese leaders.
Mr Obama spoke most directly to North Korea’s leaders, saying the internationally isolated country needs to change its ways because continuing down the same path will lead to “more broken dreams” and “more isolation”.
His blunt remarks came a day after he visited the Demilitarised Zone between North and South Korea and described the experience as akin to witnessing a “time warp” of despair.
“By now it should be clear,” he said. “Your provocations and pursuit of nuclear weapons have not achieved the security you seek, they have undermined it. Instead of the dignity you desire, you are more isolated.”
The nuclear summit unfolded as North Korea prepared for an announced satellite launch next month that the United States says amounts to a test of its rocketry.
As Mr Obama spoke of peace, tensions rose in the Korean peninsula. Seoul warned it might shoot down the North Korean rocket if it violates South Korean territory.
Speaking at Hankuk University, Mr Obama said that the international community has made progress in reducing the threat of nuclear material but says “we’re under no illusions”.
“Even as we have more work to do, we can already say with confidence that we have more nuclear weapons than we need,” he said.
“I firmly believe that we can ensure the security of the United States and our allies, maintain a strong deterrent against any threat, and still pursue further reductions in our nuclear arsenal.”
Mr Obama also reiterated his warning to Iran, which the US and its allies contend is defying its international obligations by pursuing an illicit nuclear program.
Mr Obama said he would discuss Iran in meetings later in the day with the leaders of Russia and China.
“Iran’s leaders must understand that there is no escaping the choice before it. Iran must act with the seriousness and sense of urgency that this moment demands,” Mr Obama said. “Iran must meet its obligations.”
Facing down Iran and North Korea, Mr Obama said a “new international norm” was emerging to deal with the two nations’ intransigence.
“Treaties are binding. Rules will be enforced. And violations will have consequences,” Mr Obama said. “Because we refuse to consign ourselves to a future where more and more regimes possess the world’s most deadly weapons.”
Mr Obama said the US was also moving forward with Russia to eliminate enough plutonium for about 17,000 nuclear weapons and turn it into electricity.
And he heralded an earlier agreement with Russia to reduce nuclear arsenals under the New Start Treaty, which Mr Obama called “the most comprehensive arms control agreement in nearly two decades”.
“When we’re done, we’ll have cut American and Russian deployed nuclear warheads to their lowest levels since the 1950s,” he said.
Mr Obama also prodded Russia in a new way, saying he would seek discussions with Moscow on an unprecedented front: reducing not only strategic nuclear warheads, but also tactical weapons and warheads in reserve.
He said he planned to discuss that proposition with Vladimir Putin when they meet in May.
Mr Obama was to meet later today with outgoing Russian president Dmitry Medvedev. Former leader Mr Putin returns to the presidency later this spring after winning an election held earlier this year.