Barack Obama, the US president, has said that he is pushing for "a world without nuclear weapons" and is committed to deterring nuclear proliferation.

"The danger of nuclear terrorism remains one of the greatest threats to global security. That is why in Seoul we need to keep at it," he said in the South Korean capital ahead of a two-day 53-nation summit that aims to curtail access by terror groups to radioactive material for building a nuclear bomb.

Obama also assured that the United States can further reduce its nuclear weapons stockpile while maintaining its strategic deterrent and international commitments.

Speaking at Hankuk University, Mr Obama reiterated the commitment of the US as ''the only nation to have ever used nuclear weapons'' to reducing its nuclear arms stockpile.

Obama told students that he is confident the United States and Russia can jointly reduce their stockpiles of nuclear weapons, building on the successful extension of arms control agreements known as START.

The White House said nuclear weapons reduction continues to be a priority in US relations with Russia, and that Obama will raise it with President-elect Vladimir Putin when they meet in May.

Obama would seek to follow on from the New Start (Strategic Arms Reduction Treaty) pact he struck in 2010 with outgoing Russian leader Dmitry Medvedev in 2010, he said.

Al Jazeera's Harry Fawcett, reporting from Seoul, said: "This was a speech by President Obama to unify the big themes of his visit; he used the speech to help achieve his dream of a nuclear free world."

"Sanctions and condemnation"

But Obama's strongest terms were directed toward North Korea: "By now it should be clear: Your provocations and pursuit of nuclear weapons have not achieved the respect you seek, but undermined it."

The US President said North Korea needs to change its ways because continuing down the same path will lead to "more broken dreams" and "more isolation".

"Instead of earning the respect of the world, you've been met with sanctions and condemnation. There will be no rewards for provocations. Those days are over, Obama said.

"To the leaders of Pyonyang, I say, this is the choice before you: Have the courage to pursue peace and give a better life to the people of North Korea."

Warning to Tehran

Obama echoed similar sentiments towards Iran.

"There is time to solve this diplomatically. It is always my preference to solve these issues diplomatically," he said.

"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."

Iran insists there is no military element to its programme but Western powers fear it is constructing nuclear weapons.

Obama ended his speech by praising South Korea's handling of nuclear technology in a peaceful and productive way.

Neither Iran or North Korea are participating in the 53-nation summit.

The summit agenda is to be expanded to include a wide variety of radiological materials which terrorists could use to make a dirty bomb - one that spreads radiological contamination rather than initiating a nuclear explosion

Obama flew on Sunday by helicopter to a US base on the edge of the Korean Demilitarized Zone (DMZ) to meet troops and take a first-hand look at one of the world's most heavily fortified frontiers.

Obama's visit to the DMZ also coincided with the end of the 100-day mourning period for the North's long-time leader, Kim Jong-il, who died in December.

PHOTO CAPTION

U.S. President Barack Obama delivers a speech at the Hankuk University of Foreign Studies in Seoul March 26, 2012.

Aljazeera