US President Barack Obama vowed on Monday to pursue further strategic arms cuts with Russia as part of his broader nuclear disarmament agenda even as he issued stern warnings to North Korea and Iran in their nuclear standoffs with the West.     

Speaking ahead of a global nuclear security summit in Seoul, Obama held out the prospect of new reductions in the US arsenal as he sought to rally world leaders for additional concrete steps against the threat of nuclear terrorism.     

'We can already say with confidence that we have more nuclear weapons than we need,' Obama told students at South Korea's Hankuk University.     

He pledged a new arms-control push with incoming Russian president Vladimir Putin when they meet in May. But any further reductions would face stiff election-year opposition from Republicans in Congress who already accuse him of weakening America's nuclear deterrent.     

Obama laid out his latest strategy against the backdrop of continued nuclear defiance from North Korea and Iran, twin challenges that have clouded his overall nuclear agenda as well as the summit getting under way in Seoul.     

Obama set expectations high in a 2009 speech in Prague when he declared it was time to seek 'a world without nuclear weapons'. He acknowledged at the time it was a long-term goal, but his high-flown oratory helped him win the Nobel Peace Prize.     

In Seoul on Monday, Obama made clear that he remained committed to that notion and insisted that 'those who deride our vision, who say that ours is an impossible goal that will be forever out of reach', were wrong.      

Though Obama was vague on exactly how such a vision would be achieved, he voiced confidence the United States and Russia, which reached a landmark arms-control treaty in 2009, 'can continue to make progress and reduce our nuclear stockpiles'.     

'I firmly believe that we can ensure the security of the United States and our allies, maintain a strong deterrent against any threat, and still pursue further reductions in our nuclear arsenal,' he said.     

But another arms accord with Moscow will be a tough sell to US conservatives who say Obama has not moved fast enough to modernize the US strategic arsenal, a pledge he made in return for Republican votes that helped ratify the Start treaty.     

The United States and Russia are the two biggest nuclear powers, possessing thousands of warheads between them; arsenals that arms-control advocates say are capable of destroying the world several times over.     

With US officials privately expressing concern about China's opaqueness over its growing nuclear weapons programme, Obama said he had urged the rising Asian power 'to join us in a dialogue on nuclear issues, and that offer remains open'.     

Obama also used his speech to call on North Korea, which plans a long-range rocket launch next month, to curb its nuclear ambitions or else face further international isolation.     

'And know this - there will be no more rewards for provocations.  Those days are over.  This is the choice before you,' he said, directing his comments at North Korea's leadership.     

South Korean President Lee Myung-bak urged his Chinese counterpart, Hu Jintao, in a meeting on the sidelines of the summit to try to persuade the North to cancel the launch, which Pyongyang says is merely to send a satellite into space.     

Seoul and Washington say the launch is a disguised ballistic missile test. The flightpath from a west coast launch pad will take the rocket south towards the Philippines. Two previous launches of the long-range missile have failed.     

A defence ministry official in Seoul said South Korea was drawing up a plan to shoot the rocket down if it travels into South Korean territory, the local Yonhap news agency reported.          

On Sunday, Obama appealed to China, North Korea's only major ally, to use its influence with Pyongyang to rein in its nuclear programme.      

Obama accused Iran of having taken the 'path of denial, deceit and deception' in the past but said there was still time for a diplomatic solution and that Tehran had to act with a 'sense of urgency'.     

'Time is short,' Obama said, referring to the prospects of renewed negotiations between Iran and world powers. 'Iran's leaders must understand that there is no escaping the choice before it.'      

Tehran says its nuclear programme is purely peaceful, but Israel and Western nations believe it is moving towards a nuclear bomb that could change the regional balance of power.     

Obama has urged Israel to hold off on any pre-emptive strikes on Iran's nuclear sites to give sanctions and diplomacy more time to work.

Obama came into office in 2008 declaring nuclear disarmament policy a major theme of his presidency, but momentum has slowed with the approach of the 2012 presidential election.    

He unveiled a revamped policy in 2010 renouncing development of new nuclear weapons and restricting use of those already in Washington's arsenal. He followed that up by signing the Start treaty with Russia.     

Obama secured commitments from world leaders at the inaugural 2010 Nuclear Security Summit in Washington to help keep bomb-grade material out of terrorists' hands, and independent experts say most of the pledges are being met - though many were modest in scope.     

He hopes to build on that in Seoul but is also likely to take a gentle approach with US partners like Pakistan, whose nuclear materials are considered among the biggest areas of risk because of the internal security threat posed by militants.     

Underscoring a US sense of caution, defence and national security officials have spent months debating a secret set of new options being prepared for Obama to help guide future arms-control talks. Ideas range from maintaining the status quo to reducing warheads by up to 80 per cent, an official has said.      

But the administration appears reluctant to push publicly on such a divisive issue as his re-election campaign gathers pace. – Reuters