The United States will work toward a world without nuclear missiles, president Barack Obama said in a speech today in Prague.
Delivering a speech given fresh urgency by North Korea's rocket launch hours earlier, Mr Obama made it clear the United States would still go ahead with plans to build a missile defence shield in Europe.
However, he said the driving force for this would be removed if the nuclear threat from Iran were eliminated.
Visiting the Czech capital on a trip marking his presidential debut on the world stage, Mr Obama committed himself to reducing the US nuclear arsenal, bringing the Comprehensive Test Ban Treaty into force and seeking tough penalties for countries that broke the rules on non-proliferation.
"The United States will take concrete steps towards a world without nuclear weapons," he told a cheering crowd of more than 20,000 in Hradcanske Square outside the medieval Prague Castle.
"To put an end to Cold War thinking, we will reduce the role of nuclear weapons in our national security strategy and urge others to do the same," Mr Obama said, adding: "We will seek to include all nuclear weapons states in this endeavour.
Referring directly to North Korea, he said Pyongyang should be punished for breaking the rules with its rocket launch and called on it verifiably to scrap its nuclear weapons programmes.
North Korea's latest action in defiance of US warnings touched off the most serious foreign policy crisis since Mr Obama took office on January 20th, and much of the world was watching to see how he handled it.
"Now is the time for a strong international response. North Koea must know that the path to security and respect will never come through threats and illegal weapons," he said.
North Korea said the launch was intended purely to put a satellite in orbit. The White house said the launch was "unsuccessful".
Pyongyang's action will be discussed at an emergency meeting of the UN Security Council later today.
The Czech Republic is one of two sites in eastern Europe earmarked for the planned U.S. missile shield which has angered Russia.
Mr Obama's predecessor George W. Bush said the shield was necessary to counter threats from what he called rogue states such as Iran. The new administration has been less assertive in pushing the plan but has kept it alive for now.
In Sunday's speech, Mr Obama presented Iran -- a US foe he has sought to engage diplomatically -- with a "clear choice" of halting its nuclear and ballistic missile activity or facing increased international isolation.
"As long as the threat from Iran persists, we intend to go forward with a missile defense system that is cost-effective and proven," Mr Obama said.
"If the Iranian threat is eliminated, we will have a stronger basis for security, and the driving force for missile defence construction in Europe at this time will be removed," he added. Iran says it wants nuclear technology to generate electricity, not to use for weapons as the West suspects.
Aides said Mr Obama hoped that setting out his vision for a nuclear-free world would lend credibility to Washington's efforts to resolve atomic disputes with countries such as Iran and North Korea.
Sounding a note of realism, Mr Obama said he did not expect overnight success in efforts to rid the world of nuclear arms.
"As long as these weapons exist, we will maintain a safe, secure and effective arsenal to deter any adversary, and guarantee that defence to our allies," he said.
Mr Obama said he would host a summit within a year on global nuclear security, covering issues such as preventing the smuggling of nuclear material and keeping atomic weapons out of the hands of terrorists.
On Wednesday, Mr Obama met Russian president Dmitry Medvedev on the sidelines of a Group of 20 economic summit, where the two leaders pledged to pursue a new deal to cut nuclear warheads.
The call for renewed efforts to achieve global nuclear disarmament was expected to resonate in Europe, where anti-war sentiment had traditionally been strong because of its proximity to the Soviet bloc and the front line of the Cold War.
Earlier, Gary Samore, White House coordinator for arms control, said the North Korean rocket launch meant missile defence would remain a priority.
"The North Korean test illustrates the importance of continuing to develop missile defence in order to protect ... both the country and our allies in Asia," Samore told reporters.
In Prague, Mr Obama also planned to discuss climate change and energy security with the leaders of the 27 European Union countries at a summit hosted by the Czech EU presidency.
Reuters