US President Barack Obama (left) with Russian President Dmitry Medvedev after the conclusion of their bilateral meeting at the Nuclear Security Summit in Seoul on Monday. Image Credit: AP

Seoul: US President Barack Obama vowed yesterday to pursue further nuclear arms cuts with Russia, urged China to follow suit and issued stern warnings to North Korea and Iran in their nuclear standoffs with the West.

Acknowledging the US has more warheads than necessary, 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 a few hours before a global nuclear security summit opened in Seoul.

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 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". 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 US 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 US 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 US and Russia are the two biggest nuclear powers. Obama said he wanted to take arms control talks with the Russians to a new level. "Going forward, we'll continue to seek discussions with Russia on a step we have never taken before — reducing not only our strategic nuclear warheads, but also tactical weapons and warheads in reserve," he said.

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.