The long-term outlook for keeping U.S. nuclear weapons safe and reliable is "bleak," Defense Secretary Robert Gates said Tuesday. In part, he said, that was because the United States is experiencing a brain drain in the laboratories that design and develop the world's most powerful weapons.

Gates said America's more than 5,000 nuclear weapons are now safe and secure, but he sketched out a series of concerns about the future, while stressing that nuclear weapons must remain a viable part of the U.S. strategy for deterring attack as long as other countries have them.

"Hope as we will, the power of nuclear weapons and their strategic impact is a genie that cannot be put back in the bottle — at least for a very long time," he said in remarks at the Carnegie Endowment for International Peace, a think tank that advocates the elimination of nuclear arms.

In a later question-and-answer session with his audience, Gates said he is concerned about the possibility that some Russian nuclear weapons from the old Soviet arsenal may not be fully accounted for.

"I have fairly high confidence that no strategic or modern tactical nuclear weapons have leaked" beyond Russian borders, Gates said. "What worries me are the tens of thousands of old nuclear mines, nuclear artillery shells and so on, because the reality is the Russians themselves probably don't have any idea how many of those they have or, potentially, where they are."

Gates also said that if were advising the next U.S. president, he would advocate new nuclear talks with Moscow.

"I believe we should go for another agreement with the Russians," he said. "I believe it could involve further cuts in the number of deployed warheads. I believe we do need the verification provisions. But I think it ought to be an agreement that is shorter, simpler and easier to adjust to real-world conditions than most of the strategic arms agreements that we've seen over the last 40 years."

Both presidential candidates, Republican John McCain and Democrat Barack Obama, advocate negotiating further reductions with Russia.

Gates offered a number of reasons why the United States should maintain its nuclear arsenal, including the assertion that by providing an umbrella of protection for allies like Japan and South Korea, it removes a reason for those countries to feel the need to develop their own nuclear weapons.

Echoing concerns by some congressional Republicans, Gates said there are reasons to worry about the U.S. arsenal.

"Let me first say very clearly that our weapons are safe, secure and reliable," Gates said. "The problem is the long-term prognosis — which I would characterize as bleak." He noted that the United States has not designed a new nuclear weapon since the 1980s and has not built a new one since 1992.

In his most extensive remarks on nuclear weapons since he became Pentagon chief nearly two years ago, Gates spelled out in detail his views on why nuclear weapons play a vital role in the broader U.S. defense strategy. And he called for urgent action to reverse a decline in focus on nuclear issues.

"Currently the United States is the only declared nuclear power that is neither modernizing its nuclear arsenal nor has the capability to produce a new nuclear warhead," he said. "To be blunt, there is absolutely no way we can maintain a credible deterrent and reduce the number of weapons in our stockpile without resorting to testing our stockpile or pursuing a modernization program."

The Gates remarks come amid a growing debate in national security circles over whether and how the United States should take the lead in pushing for the eventual elimination of nuclear weapons.

Gates made clear he believes that such a goal, while reasonable, cannot be realized for many years.

"We must take steps to transform from an aging Cold War nuclear weapons complex that is too large and expensive to a smaller, less costly but modern enterprise that can meet our nation's nuclear security needs for the future," Gates said.

He urged Congress to drop its opposition to a long-stymied administration proposal to develop a design for a more secure nuclear warhead, saying it could be done without actual underground nuclear testing.

"The program would reinvigorate and rebuild our infrastructure and expertise," Gates said.