"I believe the United States has a unique responsibility to act -- indeed, we have a moral obligation," Obama told students at South Korea's Hankuk University of Foreign Studies.
"I say this as president of the only nation ever to use nuclear weapons," Obama said. "I say it as a commander in chief who knows that our nuclear codes are never far from my side. Most of all, I say it as a father, who wants my two young daughters to grow up in a world where everything they know and love can't be instantly wiped out."
Obama promised to pursue further strategic arms cuts with Russia as part of an expansive nuclear disarmament agenda.
"Going forward, we'll continue to seek discussions with Russia on a step we have never taken before -- reducing not only our strategic nuclear warheads, but also tactical weapons and warheads in reserve," Obama said.
"I look forward to discussing this agenda with President [Vladimir] Putin when we will meet in May," he said.
In May, Putin returns to the post of president, which he held for eight years before spending the past four as prime minister.
In his university address, Obama warned North Korea it will further deepen its isolation if it moves forward with a plan to launch a satellite into orbit next month.
The United States and other countries say the launch would violate U.N. Security Council resolutions demanding North Korea stop launching rockets that use long-range intercontinental ballistic missile technology, like the one that would carry the satellite into space.
Washington and the Security Council have condemned such launches as a cover for developing missiles that could be used to deliver nuclear weapons. Obama said a U.S. offer of food aid could be jeopardized if the North proceeds with the launch.
"The United States has no hostile intent toward your country," Obama said, taking an opportunity "to speak directly" from South Korea to North Korean supreme leader Kim Jong Un and other North Korean leaders.
"We are committed to peace," Obama said. "And we are prepared to take steps to improve relations, which is why we have offered nutritional aid to North Korean mothers and children.
"But by now it should be clear, your provocations and pursuit of nuclear weapons have not achieved the security you seek -- they have undermined it," he said.
"Instead of the dignity you desire, you're more isolated. Instead of earning the respect of the world, you've been met with strong sanctions and condemnation. You can continue down the road you are on, but we know where that leads. It leads to more of the same -- more broken dreams, more isolation, ever more distance between the people of North Korea and the dignity and the opportunity that they deserve."
Before Obama's speech, Pyongyang said it would interpret any critical statement of its nuclear program as "a declaration of war."
South Korean President Lee Myung-bak Monday urged his Chinese counterpart, Hu Jintao, to persuade North Korea to cancel the launch.
Obama also said Iran must comply with international demands about its nuclear program.
"Iran must act with the seriousness and sense of urgency that this moment demands. Iran must meet its obligations," Obama said.