The new American president opened a town-hall style gathering with the declaration, saying he would outline details in Prague in the coming days.
"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 said, previewing a planned speech.
He was in the midst of his first European trip as president as he sought to strengthen the United States' standing in the world while working with foreign counterparts to right the troubled global economy.
Obama said the United States shares blame for the crisis, but that "every nation bears responsibility for what lies ahead — especially now."
Back home, his administration was trying to weather the fallout of another dismal monthly jobs report. The jobless rate jumped to 8.5 per cent, the highest since late 1983.
"I've come to Europe this week to renew our partnership," Obama said. "America is changing but it cannot be America alone that changes."
Obama opened the appearance — in a sports arena and before a French and German audience with many students — with several minutes of prepared remarks read from a Teleprompter. In them, he underscored the importance of the relationship between Europe and America, and he encouraged Europe to support his retooled strategy to root out terrorism in Afghanistan and Pakistan.
"I understand this war has been long. Our allies have already contributed greatly to this endeavor," Obama said. "Understand we would not deploy our own troops if this mission was not indispensable to our common security ... I understand there's doubt about his war in Europe. There's doubt even in the United States."
But he said the United States and its allies must continue to go after terrorists to ensure a safer world.
Obama bluntly said the relationship between the United States and Europe has gone adrift, with blame on both sides.
In America, he said, "there's a failure to appreciate Europe's leading role in the world."
"There have been times when America's shown arrogance and been dismissive, even derisive," Obama said.
Yet in Europe, he said, there is an anti-America attitude that seems casual but can also be insidious. He said America can be unfairly blamed for many problems.
"On both sides of the Atlantic, these attitudes have become all too common," Obama said. "They are not wise."