"As the only nuclear power to have used a nuclear weapon, the United States has a moral responsibility to act," Obama said in a keynote speech in Prague before moving on to Turkey Sunday night. "We cannot succeed in this endeavour alone, but we can lead it.
"So today, I state clearly and with conviction America's commitment and desire to seek the peace and security of a world without nuclear weapons," he said in a warmly received speech to 30,000 people at picturesque Prague Castle.
Obama, in the Czech Republic for an EU-US summit, said he wanted an immediate end to nuclear tests, confirmed he would seek Senate approval of the Comprehensive Test Ban Treaty and would hold a global summit on nuclear security within the next year.
"I am not naive, this goal (of a nuclear free world) will not be reached quickly, perhaps not in my lifetime," he added, branding nuclear weapons stockpiles as the most dangerous legacy of the Cold War.
Doing nothing to stop the spread of nuclear weapons would be to cede to a "deadly adversary" of fatalism, he warned.
"One nuclear weapon exploded in one city -- be it New York or Moscow, Islamabad or Mumbai, Tokyo or Tel Aviv, Paris or Prague -- could kill hundreds of thousands of people," he said.
The euphoric mood in Prague, which saw late-night revellers mingling with young Obama fans, contrasted with the president's sombre tone, a message lent more poignancy by North Korea's rocket launch over Japan.
Obama said Pyongyang must pay a price and called for a strong international response at an emergency UN Security Council meeting.
"Rules must be binding, violations must be punished, words must mean something," he said.
"This provocation underscores the need for action -- not just this afternoon at the UN Security Council, but in our determination to prevent the spread of these weapons," Obama said.
The US president also vowed to move forward with a controversial plan to base a missile defence shield in central Europe which has upset Moscow, saying the threat posed by Iran remained real.
"Let me be clear: Iran's nuclear and ballistic missile activity poses a real threat, not just to the United States, but to Iran's neighbours and our allies," Obama said.
"The Czech Republic and Poland have been courageous in agreeing to host a defence against these missiles. As long as the threat from Iran persists, we intend to go forward with a missile defence system that is cost-effective and proven."
The shield was officially devised by former president George W. Bush to defend against long-range ballistic missiles possibly fired by "rogue states" such as Iran, but Moscow views it as a threat to Russian security.
Before leaving for Turkey, Obama met with former Czech president and celebrated playwright Vaclav Havel, who warned Obama of the dangers to his presidency of rising expectations at home and abroad.
"There is a danger that people may end up thinking that he has betrayed them, that he raised their hopes too far," said Havel.
Obama also held talks with Czech President Vaclav Klaus and outgoing Prime Minister Mirek Topolanek.
A Czech news agency reported later Sunday a deal to replace Topolanek -- who stirred stateside controversy by labelling Obama's economic policies "the road to Hell" -- with a temporary prime minister.
Subject to ratification, Jan Fischer would also assume the EU's chairmanship for the next three months, pending elections in October.
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