PRAGUE (Agencies) - US President Barack Obama pledged Sunday to lead a quest for a world purged of nuclear weapons, denouncing 'fatalism over proliferation. Warning the prospect of a terrorist with a nuclear device was the 'most immediate and extreme threat to global security, Obama announced an initiative to cut stockpiles, curtail testing and secure loose nuclear material. 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. We cannot succeed in this endeavour alone, but we can lead it. So today, I state clearly and with conviction Americas 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 town 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 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: Irans nuclear and ballistic missile activity poses a real threat, not just to the United States, but to Irans 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 project 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.