Seoul, South Korea (CNN) -- President Barack Obama on Monday emphasized his vision for a nuclear-free world and warned that North Korea and Iran should shun nuclear weapons or face tough action from the international community.
Obama, who is in Seoul for a nuclear summit, will meet his Russian and Chinese counterparts Monday.
Earlier, during a speech to students at Hankuk University in South Korea, he warned North Korea that if it moves forward with a planned test-firing of a long-range missile, it will further deepen its isolation.
Obama said sanctions have led to the "slowing" of Tehran's nuclear program. But it remains a concern, and will be a topic of discussion later with leaders of China and Russia.
While the president didn't specify the course of action if Iran does not comply with international demands and produces nuclear weapons, he left little leeway for Tehran's leaders.
"Iran must act with the seriousness and sense of urgency that this moment demands. Iran must meet its obligations," Obama said.
The president also directed remarks at the leaders of neighboring North Korea.
He said the United States has "no hostile intent toward your country" and is "committed to peace."
But Pyongyang needs to realize that years of sanctions and condemnation show that its existing nuclear strategy isn't working, Obama said, adding "there will be no more rewards for provocations."
"You can continue down the road you are on, but we know where that leads," Obama said. "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 opportunity they deserve."
The president ended his speech by predicting that Koreans, North and South, will someday be "united and free."
"The day all Koreans yearn for will not come easily or without great sacrifice. But make no mistake, it will come. And when it does, change will unfold that once seemed impossible," he said.
Prior to Obama's speech, Pyongyang said it will see any critical statement of its nuclear program as "a declaration of war." His remarks follows last week's announcement by North Korea that it is planning to carry out a rocket-powered satellite launch in April.
Using ballistic missile technology, however, is in violation of U.N. Security Council Resolution 1874 and against a deal struck with the United States earlier this month that it would not carry out nuclear or missile tests in return for food aid.
North Korea and Iranian issues are overshadowing the message of international cooperation for the summit, which is bringing together top officials from 54 countries, including China and Russia.
The nuclear summit will be the second under Obama, after he hosted a meeting in Washington in 2010. He initiated the biennial summit after presenting his vision of a nuclear-free world in Prague in April 2009.
In his speech Monday, Obama said he's committed to further cutting his nation's stockpile of strategic nuclear weapons, but also tactical weapons and warheads.
"I believe the United States has a unique responsibility to act," Obama said. Later in the speech, he added, "We have more nuclear weapons than we need."