President Barack Obama unveiled his vision for a world free of nuclear weapons in a major speech Sunday in the Czech capital.

Speaking to a crowd of some 30,000 under the spiky spires of Prague Castle, the US leader pledged that Washington will take "concrete steps" to reduce US nuclear weapon stockpiles and underscored the urgent need to bring into force the Comprehensive Nuclear Test Ban Treaty.

"Some argue that the spread of these weapons cannot be checked — that we are destined to live in a world where more nations and more people possess the ultimate tools of destruction," Mr. Obama told the crowd in his key address during his maiden diplomatic tour of Europe as president. But criticizing such fatalistic attitudes as a "deadly adversary," he added, "If we say to ourselves that the spread of nuclear weapons is inevitable, we are saying that the use of nuclear weapons is inevitable."

Obama said Washington would host a summit by the end of the year to discuss "locking down" loose fissile materials.

Obama's speech came hours after news North Korea tried and failed to launch a satellite into orbit on a long-range rocket that can carry nuclear warheads.

Obama joined a wave of international criticism, saying Pyongyang had "broken the rules" and must be forced to change. "Now is the time for a strong international response," he said.

If Iran backs down ...

Obama also referred to Iran, saying the US will go ahead with a plan to build a missile defense shield, "as long as the threat from Iran persists."

The US leader, however, added, "If the Iranian threat is eliminated, we will have a stronger basis for security, and the driving force for missile construction in Europe will be removed."

Key components of a missile defense system (a radar tracking station) would be based in the Czech Republic and (10 interceptor missiles) in neighboring Poland.

At a press conference later, Czech Prime Minister Mirek Topolanek said Obama's speech made it clear the US has every intention to build the antimissile shield.

But analysts say the system's future is shaky as Washington looks to "reset" relations with Moscow, which opposes the system.

"The United States needs to get relations with Russia back on track; it needs Russia's help on a range of issues," explains Lawrence Korb, a former Reagan defense official and now a senior fellow at the Center for American Progress, a liberal think tank in Washington.

Why the US needs Russian help

At Obama's first meeting with Russian President Dmitry Medvedev in London on Wednesday, both agreed on fast-track negotiations to slash their countries' nuclear stockpiles by about a third.

Besides nuclear reduction, Mr. Korb says the US needs Russian help on Iran's nuclear program and on finding new supply routes into Afghanistan. "The missile shield is really a secondary issue," says Korb.

Czechs and others in Eastern Europe, however, are watching nervously as Washington warms to Moscow. The war with Georgia last summer and second gas spat with Ukraine last winter have upped fears of a resurgent Russian bear.

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