Barack Obama, the US president, has said that he is pushing for "a world without nuclear weapons” and is committed to deterring nuclear proliferation, while on a visit to Seoul for a two-day 53-nation nuclear summit that is under way."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 before the summit focussing on reducing access by terror groups to radioactive material to build a nuclear bombObama 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 before the summit on Monday, 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 continued to be a priority in US relations with Russia, and that Obama would 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 needed to change its ways because continuing down the same path would 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.”Hu Jintao, the Chinese president, has expressed "serious concern” about North Korea’s planned rocket launch during a meeting with US counterpart Barack Obama on the sidelines of the summit, a US official said on Monday."The two leaders agreed to co-ordinate closely in responding to this potential provocation and registering our serious concern to the North Koreans and, if necessary, consider what steps need to be taken following a potential satellite launch,” Ben Rhodes, a deputy national security adviser, said.Warning to TehranObama 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.