U.S. President Barack Obama delivered his speech Sunday in Prague where he is also attending a summit between the United States and the 27-member European Union. (Herbert Knosowski/AP Photo)

U.S. President Barack Obama in Prague on Sunday laid out his vision for a world free from the threat of nuclear weapons, calling them "the most dangerous legacy of the Cold War."

In a speech before a cheering crowd in the Czech capital, Obama said he would begin reducing the U.S. arsenal, work on a new nuclear weapons treaty with Russia and prevent the spread of nuclear weapons technology.

He urged an international effort to secure all nuclear materials within the next four years.

"We will set new standards, expand our co-operation with Russia, pursue new relationships to lock down these sensitive materials," he said. "We must also build on our efforts to break up black markets, detect and intercept materials in transit, and use financial tools to disrupt this dangerous trade, because this threat will be lasting."

Obama made the comments while attending a meeting with the 27 leaders of the European Union, who were expected to discuss climate change and energy security.

"As the only nuclear power to have used a nuclear weapon, the United States has a moral responsibility to act," he said.

He said the U.S. would host a summit within the next year on reducing and eventually eliminating nuclear weapons. Obama said he wants to see a new nuclear weapons treaty with Russia, and all nuclear nations.

"[Russian] President Dmitry Medvedev and I began this process in London and will seek a new agreement by the end of this year that is legally binding and sufficiently bold. And this will set forth further cuts and we will seek to include all nuclear weapons states in this endeavour," he said.

The U.S. president also said, however, that his country will proceed with development of a missile defence system in Europe, as long as there is a threat of Iran developing nuclear weapons.