China has told the United States it has "serious concerns" about its ally North Korea's planned rocket launch, a US official says.

Chinese president Hu Jintao spoke with US president Barack Obama on Monday as the 2012 Seoul Nuclear Security Summit opened in South Korea.

Nuclear defiance from North Korea and Iran is expected to dominate the overall nuclear agenda at the summit, with numerous nations strongly criticising North Korea's rocket launch set for mid-April.

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The nuclear-armed North insists it will only put a satellite into orbit, while its opponents say it will test missile technology that could deliver a warhead.

China, the North's sole major ally and its biggest trade partner and aid provider, is seen as one of the few nations that can influence the regime.

Senior White House aide Ben Rhodes said Mr Hu indicated to Mr Obama that he took the North Korean nuclear standoff very seriously and was registering his concern with Pyongyang.

"The two leaders agreed to coordinate 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," Mr Rhodes said.

The nuclear security summit in Seoul has officially opened, with Prime Minister Julia Gillard amongst 52 world leaders in attendance.

Ahead of the conference, Ms Gillard told the audience of Korean business leaders and students that the friendship between both countries is growing.

But she says it is inevitable that North Korea's planned nuclear launch will dominate discussions.

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Nuclear disarmament

Mr Obama has vowed to pursue further nuclear arms cuts with Russia and urged China to follow suit.

Acknowledging the United States has more warheads than necessary, Mr 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," Mr Obama told students at South Korea's Hankuk University ahead of the Seoul summit.

Mr 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, Mr 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 Mr 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 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 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.

Mr 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.

South Korea's president Lee Myung-Bak greets Prime Minister Julia Gillard in Seoul. ( AFP: Saul Loeb )

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

Mr Obama also used his speech to call on North Korea 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.

Mr Obama says the destitute North could be hit with tighter sanctions if it goes ahead with the rocket launch, but experts doubt China will back another UN Security Council resolution against North Korea.

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

Even though two previous launches of the long-range missile have failed, Washington says the North's missile program is progressing quickly and that the American mainland could come under threat within five years.

ABC/wires