Declaring the future of mankind at stake, President Barack Obama on Sunday said all nations must strive to rid the world of nuclear arms and that the U.S. had a "moral responsibility" to lead because no other country has used one.

A North Korean rocket launch upstaged Obama''s idealistic call to action, delivered in the capital of the Czech Republic, a former satellite of the Soviet Union. But Obama dismissed those who say the spread of nuclear weapons, "the most dangerous legacy of the Cold War," cannot be checked.

"This goal will not be reached quickly — perhaps not in my lifetime," he told a cheering crowd of more than 20,000 in the historic square outside the Prague Castle gates. We "must ignore the voices who tell us that the world cannot change. We have to insist, ''Yes, we can.''"

Few experts think it''s possible to completely eradicate nuclear weapons, and many say it wouldn''t be a good idea even if it could be done. Even backward nations such as North Korea have shown they can develop bombs, given enough time.

But a program to drastically cut the world atomic arsenal carries support from scientists and lions of the foreign policy world. Obama embraced that step as his first goal and chose as the venue for his address a nation that peacefully threw off communism and helped topple the Soviet Union, despite its nuclear power.

But he said his own country, with its huge arsenal and its history using two atomic bombs against Japan in 1945, had to lead the world. He said the U.S. has a "moral responsibility" to start taking steps now.

"To reduce our warheads and stockpiles, we will negotiate a new Strategic Arms Reduction Treaty with the Russians this year," he promised.