STRASBOURG - U.S. President Barack Obama called on Friday for a world without atomic weapons and urged allies to stand firm against Iran's nuclear ambitions and a planned missile launch by North Korea.

On his first major foreign trip since he took office in January, Obama held talks with French President Nicolas Sarkozy in eastern France and took questions from young French and Germans before a NATO summit hosted by France and Germany.

A crowd mobbed him, and a woman kissed him, as he went into talks with Sarkozy, a day after helping broker a deal to combat the global economic crisis at the Group of 20 summit in London.

Even with the Cold War now over, the spread of nuclear weapons, or the theft of nuclear material, could lead to the extermination of any city on the planet, Obama told the young French and Germans gathered in the French city of Strasbourg.

This weekend in Prague I will lay out an agenda to seek the goal of a world without nuclear weapons, he said, referring to talks with European Union leaders on Sunday.

He gave no further details but his comments followed talks with Russian President Dmitry Medvedev in London on Wednesday at which the two leaders agreed to seek a deal to cut the nuclear arsenals their countries built up during the Cold War.

Taking questions from the German and French youths on issues ranging from the economic crisis to bringing a dog to the White House, he said a nuclear arms deal would increase Washington's moral authority in dealings over North Korea and Iran.

FIRM LINE ON IRAN, NORTH KOREA

Obama also told a joint news conference with Sarkozy that Iran must not be allowed to develop nuclear weapons, although he ahs reached out to Tehran since he took office, and warned nuclear-armed North Korea that the international community would take steps if it went ahead with its planned missile launch.

We cannot have a nuclear arms race in the Middle East, Obama said in reference to Iran's nuclear program, which the West believes is a cover for producing a nuclear bomb and Tehran says is solely for peaceful energy use.

North Korea has said it will send a satellite into space between April 4 and 8, and that it has the right to do so as a part of a peaceful space program.

The launch is widely seen abroad as a disguised long-range missile test despite U.S. warnings it would be provocative and breach United Nations resolutions.

Obama said that if Pyongyang launched the missile, the international community would take appropriate steps to show North Korea it cannot threaten the safety and stability of other countries with impunity.

The response so far from the North Koreans has been not just unhelpful but has resorted to the sort of language that has led to North Korea's international isolation in the international community for a very long time, he said.

He gave no indication of what steps Washington and other nations would take in response.

Obama also discussed plans to improve ties with Russia. The two sides have lingering differences over some issues, including Moscow's brief war last year with NATO-aspirant Georgia.

He said the United States wanted to work with Russia but, referring to the Cold war when Moscow dominated eastern Europe, it could not go back to the old ways of doing business.

It is important for NATO allies to engage Russia, and to recognize that they have legitimate interests. In some cases we have common interests, but we also have some core disagreements, Obama said.

I think we should be in a dialogue with them about how we can maintain stability, while respecting the autonomy and independence of all countries in Europe, west, east, central, wherever they are.

NATO has announced plans to resume formal ties with Russia, frozen after its war with Georgia, saying cooperation with Moscow is essential on many global security matters, including Tehran's nuclear program and the war in Afghanistan.

Germany Russia Nuclear weapons Dmitry Medvedev