US President Barack Obama raised the prospect of further reducing his country's nuclear arsenal as he sought to rally leaders against threats from North Korea and Iran.

Mr Obama pledged to work towards a new arms-control push with Russian President Vladimir Putin when they meet in May.

"We can say with confidence we have more nuclear weapons than we need," Mr Obama told students at South Korea's Hankuk University before a global nuclear security summit in Seoul.

Mr Obama laid out his strategy against the backdrop of defiance from North Korea and Iran.

In Seoul, Mr Obama said he was still committed to a world without nuclear weapons, saying "those who deride our vision, who say that ours is an impossible goal that will be forever out of reach", were wrong.

He said the US and Russia, which reached a landmark arms-control treaty in 2010, "can continue to make progress and reduce our nuclear stockpiles".

He added: "I firmly believe 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."

A new arms accord will be a tough sell to US conservatives who say Mr 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 agreement with Russia in 2010.

Mr Obama said: "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."

With US officials expressing concern about China's growing nuclear weapons programme, Mr Obama said he urged it "to join us in a dialogue on nuclear issues".

He also sent a warning to North Korea, which plans a long-range rocket launch next month. "Know this – there will be no more rewards for provocations. Those days are over," he said.

Chinese President Hu Jintao indicated he was registering his concern with Pyongyang, a senior White House aide said. China is the North's only major ally.

The North says the rocket will send a satellite into space, but South Korea and the US say it is a ballistic missile test.

While two previous launches of the missile have failed, the US says the American mainland could be under threat within five years.

Mr Obama also accused Iran of taking the "path of denial, deceit and deception" but said there was time for a diplomatic solution.

Referring to the possibility of negotiations, he said: "Time is short. Iran's leaders must understand there is no escaping the choice before it.".

He also met Russia's outgoing leader, Dmitry Medvedev, with missile defence in Europe, Iran and Syria topping the agenda.

Mr 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 UN Security Council condemnation by vetoing two resolutions.