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 don't. Failure, he said Monday, would mean a future without dignity, respect or hope for its people.

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 U.S. ability to deter aggression AP reported.

As Obama spoke of peace in the midst of an international nuclear summit, tensions rose in the Korean peninsula. Seoul warned it might shoot down a North Korean rocket carrying a satellite if it violates South Korean territory. The United States maintains the launch amounts to a test of North Korea's rocketry.

In unusually personal terms, Obama said he spoke of his wish for further nuclear reductions as the president of the only nation ever to use 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.

Obama also met on the sidelines of the summit with outgoing Russian President Dmitry Medvedev. Later he planned to sit down with Chinese President Hu Jintao, whom he was expected to press for help in dealing with North Korea.

Following their meeting, Obama and Medvedev acknowledged continued tensions between their countries on key issues, including Syria and missile defense.

Still, Obama aimed his most stern remarks 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 Demilitarized 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."

Speaking at Hankuk University, 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."

Obama also reiterated his warning to Iran, which the U.S. and its allies contend is defying its international obligations by pursuing an illicit nuclear program. 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," Obama said. "Iran must meet its obligations."

Facing down Iran and North Korea, 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," Obama said. "Because we refuse to consign ourselves to a future where more and more regimes possess the world's most deadly weapons."

Obama said the U.S. 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 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.

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 former leader Vladimir Putin, who will return to the presidency later this spring after winning an election held earlier this year, when they meet in May.

After his meeting with Medvedev, Obama said the U.S. and Russia have "more work to do" to bridge their differences, including their approach to violence in Syria. The U.S. has sharply criticized Russia for opposing U.N. Security Council action calling on Syria's president to leave power.

Despite their differences, Medvedev says the relationship between the U.S. and Russia has reached its "best level".