Obama vows to pursue further nuclear cuts with Russia

US President Barack Obama vowed on Monday to pursue further nuclear arms cuts with Russia, saying the United States has more warheads than necessary, and 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 and the summit in Seoul. He 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, Obama made clear that he was committed to that notion, saying "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 2010, "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 U.S. conservatives who say Obama has not moved fast enough to modernise 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 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.

Pyongyang says the rocket will send a satellite into space, but Seoul and Washington say it is a ballistic missile test.

Two previous launches of the long-range missile have failed. The flightpath from a west coast launch pad will take the rocket south towards the Philippines.

The defence ministry in Seoul said South Korea was drawing up a plan to shoot down any components that may crash into South Korean territory. The ministry added that the main body of the rocket was being positioned into place for the launch.

On Sunday, Obama appealed to China, North Korea's only major ally, to use its influence with Pyongyang to rein in its nuclear programme. Chinese President Hu Jintao said the North Korean issue was "very complicated and sensitive".

"We do not hope to see a reversal of the hard-won momentum of relaxation of tension on the peninsula," Hu said, according to the official Xinhua news agency. Obama told Hu before going into a closed meeting on Monday that North Korea and Iran were "obviously of great importance".

"We both have an interest in making sure that international norms surrounding non-proliferation, preventing destabilising nuclear weapons, is very important," he said.

Earlier on Monday, 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.

The US president also met Russia's outgoing leader, Dmitry Medvedev, on the sidelines of the summit, with missile defence in Europe, Iran and the conflict in Syria topping their agenda.

Medvedev said he supported peace envoy Kofi Annan's mission to end fighting in Syria. Russia and China have shielded Syrian President Bashar al-Assad from U.N. Security Council condemnation by vetoing two resolutions over the bloodshed.

"We together with the US president maintain that (Annan's mission) is a good way to reach at least an initial point of settlement (of the crisis) and open the road for communication between various groups of society in Syria," he said.