US President Barack Obama urged North Korea's leaders Monday to "have the courage to pursue peace" and said Iran was running out of time to end its own nuclear standoff with the world.

Obama, in an unusually direct appeal to Pyongyang's new leaders, said Washington had "no hostile intent" and was ready to take steps to improve relations.

"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 in a major speech at Hankuk University in Seoul.

"Instead of the dignity you desire, you are more isolated. Instead of earning the respect of the world, you have been met with strong sanctions."

If the North's leaders continue down the same road, it will lead to "more broken dreams, more isolation and ever more distance between the people of North Korea and the dignity and opportunity they deserve", he said.

Obama, who is visiting South Korea for a 53-nation nuclear security summit starting later Monday, has sharply criticised North Korea's plan for a rocket launch next month.

The North says it will launch a peaceful satellite. The United States and other nations say a disguised missile test would breach UN resolutions and a US-North Korea deal reached last month.

That deal -- offering major food aid in return for a partial nuclear freeze and a missile test moratorium -- is now in jeopardy.

There will be "no more rewards for provocations. Those days are over," Obama told the North's leaders, urging them to "have the courage to pursue peace and give a better life to the North Korean people".

The untested Kim Jong-Un was proclaimed "great successor" after his father Kim Jong-Il died in December. But Obama said Sunday it was unclear who was "calling the shots" under the new young leader.

Obama stressed Iran -- suspected of pursuing atomic weapons -- has the right to peaceful nuclear energy but time and again it had taken "the path of denial, deceit and deception".

"There is time to solve this diplomatically, but time is short," said Obama, who will discuss the issue with the leaders of Russia and China late Monday.

"Iran must act with the seriousness and sense of urgency that this moment demands. Iran must meet its obligations."

Obama in 2009 outlined his vision of a world without nuclear weapons and chaired the first nuclear security summit in 2010, dedicated to keeping plutonium and highly enriched uranium (HEU) out of the hands of terrorists.

Since then, the US leader said, nations like Kazakhstan had moved nuclear material to more secure locations. Mexico and Ukraine had removed all HEU from their territory.

"All told, thousands of pounds of nuclear material have been removed from sites around the world -- deadly material that is now secure and can never be used against a city like Seoul," he said.

Global authorities were also breaking up black markets, with countries such as Georgia and Moldova seizing HEU from smugglers.

Obama, who ordered the raid that killed Al-Qaeda leader Osama bin Laden last year, said that with his death and other blows to the group "a terrorist organisation that has actively sought nuclear weapons is now on the path to defeat".

But there was still much to be done. Just the smallest amount of plutonium could kill hundreds of thousands and spark a global crisis.

"The danger of nuclear terrorism remains one of the greatest threats to global security," Obama said.

Obama said he expects dozens of nations to announce they had fulfilled the promises made two years ago, during the two-day Seoul follow-up summit.

He highlighted the United States moving forward with Russia to eliminate enough plutonium for about 17,000 nuclear weapons.

The two countries were reducing atomic arsenals under the New START Treaty.

Obama said Washington would also seek talks with Russia on a new step -- reducing not only strategic nuclear warheads, but also tactical weapons and warheads in reserve.

Obama said he would discuss the issue with Russia's incoming President Vladimir Putin when they meet in May.

"I'm confident that, working together, we can continue to make progress and reduce our nuclear stockpiles," he said.