26 March 2012 Last updated at 01:39 GMT 
US President Barack Obama has been holding meetings ahead of the Seoul summit
The comments come as President Obama addresses a summit aimed at reducing the risk of nuclear terrorism.
The meeting in Seoul involves representatives from some 50 countries.
The New Start (Strategic Arms Reduction Treaty) deal agreed between Washington and Moscow aims to replace its lapsed predecessor, Start.
It trims US and Russian nuclear arsenals to 1,550 deployed nuclear warheads - a cut of about 30% from a limit set previously.
The treaty would also allow each side visually to inspect the other's nuclear capability, with the aim of verifying how many warheads each missile carries.
In addition, there will be legally binding limits on the number of warheads and missiles that can be deployed on land, on submarines, and on bombers, at any one time.
Rocket warning Earlier, US President Barack Obama warned North Korea its planned long-range missile launch would achieve nothing and increase its isolation.
Pyongyang says it is preparing to launch a long-range missile which it says will put a satellite in orbit.
Mr Obama was speaking after talks in Seoul with South Korean President Lee Myung-bak, ahead of the summit.
The two leaders said North Korea risked further sanctions and isolation if it did not cancel its launch plans.
The launch will contravene an agreement Pyongyang reached last month which would have seen it receive food aid in exchange for a partial freeze on nuclear activities and an end to ballistics tests.
The North said the launch - between 12 and 16 April - would mark the 100th birthday of its late Great Leader Kim Il-sung.
Intrusive process Despite lofty announcements it may prove difficult to achieve significant progress at the summit, says the BBC's Jonathan Marcus.
The summit agenda is to be expanded to include a wide variety of radiological materials which terrorists could use to make a dirty bomb - one that spreads radiological contamination rather than initiating a nuclear explosion
But experts say there is unlikely to be agreement on converting all nuclear power stations to use low-enriched fuel.
Nor will there be agreement on common standards for nuclear security.
Some countries see this whole process as highly intrusive.
And there is still no common appreciation of the level of threat posed by nuclear terrorism, our correspondent says.
US President Barack Obama has been holding meetings ahead of the Seoul summit US President Barack Obama will discuss further nuclear arms cuts with newly-elected Russian President Vladimir Putin in May, a US official has said.
The official said Mr Obama would seek to follow on from the New Start treaty he struck in 2010 with outgoing Russian leader Dmitry Medvedev in 2010. The comments come as President Obama addresses a summit aimed at reducing the risk of nuclear terrorism.
The meeting in Seoul involves representatives from some 50 countries.
The New Start (Strategic Arms Reduction Treaty) deal agreed between Washington and Moscow aims to replace its lapsed predecessor, Start.
It trims US and Russian nuclear arsenals to 1,550 deployed nuclear warheads - a cut of about 30% from a limit set previously.
The treaty would also allow each side visually to inspect the other's nuclear capability, with the aim of verifying how many warheads each missile carries.
In addition, there will be legally binding limits on the number of warheads and missiles that can be deployed on land, on submarines, and on bombers, at any one time.
Rocket warning Earlier, US President Barack Obama warned North Korea its planned long-range missile launch would achieve nothing and increase its isolation.
Pyongyang says it is preparing to launch a long-range missile which it says will put a satellite in orbit.
Mr Obama was speaking after talks in Seoul with South Korean President Lee Myung-bak, ahead of the summit.
The two leaders said North Korea risked further sanctions and isolation if it did not cancel its launch plans.
The launch will contravene an agreement Pyongyang reached last month which would have seen it receive food aid in exchange for a partial freeze on nuclear activities and an end to ballistics tests.
Continue reading the main story 
Ever since the collapse of the Soviet Union in 1991, there have been fears about nuclear materials falling into the hands of terrorists and extremists.
In the United States, such concerns took on added significance in the wake of the 9/11 attacks on New York and Washington. Washington and its key allies - like the UK - are convinced that al-Qaeda is seeking to obtain material for a nuclear or radiological bomb.
In April 2010, President Barack Obama convened a summit in Washington that set the ambitious goal of securing all vulnerable nuclear materials worldwide within four years. Some progress has been made - for example Chile returned highly enriched uranium to the US; Kazakhstan has moved spent fuel to a secure depot; and Ukraine has transferred fissile material to a Russian storage site.
But progress has been patchy, in part because the initial goal set in 2010 was vague without a detailed timeline or work plan. Now two years on, this Seoul summit will try to create new momentum.
Analysis
In the United States, such concerns took on added significance in the wake of the 9/11 attacks on New York and Washington. Washington and its key allies - like the UK - are convinced that al-Qaeda is seeking to obtain material for a nuclear or radiological bomb.
In April 2010, President Barack Obama convened a summit in Washington that set the ambitious goal of securing all vulnerable nuclear materials worldwide within four years. Some progress has been made - for example Chile returned highly enriched uranium to the US; Kazakhstan has moved spent fuel to a secure depot; and Ukraine has transferred fissile material to a Russian storage site.
But progress has been patchy, in part because the initial goal set in 2010 was vague without a detailed timeline or work plan. Now two years on, this Seoul summit will try to create new momentum.
The North also agreed to allow UN inspectors in, the US said.
The invitation comes three months after Kim Jong-un came to power following the death of his father, Kim Jong-il.The North said the launch - between 12 and 16 April - would mark the 100th birthday of its late Great Leader Kim Il-sung.
Intrusive process Despite lofty announcements it may prove difficult to achieve significant progress at the summit, says the BBC's Jonathan Marcus.
The summit agenda is to be expanded to include a wide variety of radiological materials which terrorists could use to make a dirty bomb - one that spreads radiological contamination rather than initiating a nuclear explosion
But experts say there is unlikely to be agreement on converting all nuclear power stations to use low-enriched fuel.
Nor will there be agreement on common standards for nuclear security.
Some countries see this whole process as highly intrusive.
And there is still no common appreciation of the level of threat posed by nuclear terrorism, our correspondent says.
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