U.S. President Barack Obama is appealing to other nations to join in an effort to rid the world of nuclear weapons. In Prague, the Czech capital, Mr. Obama said the United States will lead the effort, but cannot do it alone.

The president went straight to the European public to urge support for a difficult task. He told a crowd of about 20,000 people in a city square near Prague castle that it is time for the nations of the world to join in a common cause. "Today, I state clearly and with conviction, America's commitment to seek the peace and security of a world without nuclear weapons," he said.

He said all states with nuclear weapons should be involved. But he said it is clearly fitting that the United States takes the initiative. "As the only nuclear power to have used a nuclear weapon, the United States has a moral responsibility to act. We cannot succeed in this endeavor alone, but we can lead it, we can start it," he said.

He said the process should have three components: reduce existing nuclear stockpiles, prevent further proliferation, and protect vulnerable nuclear materials to keep them from falling into the wrong hands.

The president spoke just hours after North Korea defied the international community and launched a long-range rocket. He called the launch a provocation, and said it underscores the need for action. "Rules must be binding. Violations must be punished. Words must mean something. The world must stand together to prevent the spread of these weapons," he said.

Mr. Obama also spoke about Iran's nuclear intentions, saying they pose a real threat. He said that is why the United States has proposed a missile-defense system with components in Poland and the Czech Republic. "As long as the threat from Iran persists, we will go forward with a missile defense system that is cost effective and proven. If the Iranian threat is eliminated we will have a stronger basis for security and the driving force for missile defense construction in Europe will be removed," he said.

Officially, the president came to Prague to meet with leaders of the European Union, since the Czech Republic currently holds the revolving E.U. presidency.

European Commission head Jose Barroso said they all agreed North Korea's launch demands a response from the international community. "It is clear in our joint statement that North Korea cannot realize either international acceptance or economic development linked to the international system until it ceases its threatening behavior," he said.

The U.S. - E.U. summit also focused on climate change. Barroso told reporters President Obama offered his support for Turkey's bid to join the European Union.

Turkey is the final stop on Mr. Obama's trip to Europe. He will address the Turkish parliament and meet with the country's leaders Monday in Ankara before heading to Istanbul for a roundtable discussion with students.