PRAGUE - US President Barack Obama yesterday set out his vision for a world free of nuclear weapons, vowing to involve all states with atomic weapons in the process of reducing arsenals.

In an open-air speech in Prague, Obama said that if the nuclear threat from Iran were eliminated, this would remove the driving force for building anti-missile defences.

Visiting the Czech capital on a one-week trip marking his presidential debut on the world stage, 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 US will take concrete steps towards a world without nuclear weapons," he told a cheering crowd.

He said North Korea had broken the rules with a rocket launch earlier yesterday and called on Pyongyang to abandon its efforts to acquire nuclear weapons.

North Korea said the launch was intended purely to put a satellite in orbit.

Its action was to be discussed at an emergency meeting of the UN Security Council late yesterday.

The Czech Republic is one of two sites in Eastern Europe for a planned US missile shield that has angered Russia.

Washington says the shield is necessary to counter threats from what it calls rogue states such as Iran.

"As long as the threat from Iran persists, we will go forward with the missile system," Obama said.

"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."

Thousands of Czechs and foreign tourists crowded into Hradcanske Square outside the medieval Prague Castle to hear Obama speak. - Reuters