US Defense Secretary Robert Gates called Tuesday for the modernization of the US nuclear arsenal to strengthen deterrence at a time when Russia and China are upgrading their nuclear weapons.

"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 weapon," he said in a speech to the Carnegie Endowment for Peace.

"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," he said.

Gates said the development of a new so-called Reliable Replacement Warhead was needed to ensure the long-term viability of the US stockpile and to revive a nuclear industrial base that is in decline.

To add teeth to deterrence, Gates said, the United States is pursuing new technologies that can identifiy "forensic signatures" of any nuclear material used in an attack and trace it back to its source.

He said the United States would hold "fully accountable" any state, terrorist group, non-state actor or individual that supports or enables terrorist efforts to obtain or use weapons of mass destruction.

Congress cut funding this year for the Reliable Replacement Warhead program, and candidates from both parties have called for deeper cuts in the US arsenal.

Critics of the Reliable Replacement Warhead program see it as contrary to US commitments under the 1968 nuclear Non-Proliferation Treaty to eventually eliminate nuclear weapons.

The United States and Russia agreed in 2002 to reduce "operationally deployed" warheads to around 1,700-2,200 by 2012.

As of January 1, 2008, the United States had about 5,400 warheads in its nuclear arsenal, about 4,075 of which were operational and most of the others held in reserve, according to the Bulletin of Atomic Scientists.

Gates acknowledged that neither Russian nor China were adversaries.

But he argued that the "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."

"There is no way to ignore efforts by rogue states such as North Korea and Iran to develop and deploy nuclear weapons, or Russian and Chinese strategic modernization programs," he said.

"As long as other states have or seek nuclear weapons -- and can potentially threaten us, our allies and our friends -- then we must have a deterrent capability that makes it clear that challenging the United States in the nuclear arena -- or with other weapons of mass destruction -- could result in an overwhelming, catastrophic response," he said.

Russia, unable to maintain its conventional forces at Cold War levels, is increasingly reliant on its nuclear forces and maintains a fully functional capacity to manufacture significant numbers of nuclear warheads, he said.

"China is also expanding its nuclear arsenal. It has increased the number of short, medium and long-range missiles -- and pursued new land, sea, and air-based systems that can deliver nuclear weapons," he said.

Gates said the US nuclear umbrella enables US allies in Europe and the Pacific who worry about Iran and North Korea "to continue to rely on our nuclear deterrent rather than to develop their own."

He said the nuclear weapons in the existing stockpile are safe, secure and reliable but that the long-term prognosis was "bleak."

Warning of a brain drain, he said, "No one has designed a new nuclear weapon in the United States since the 1980s, and no one has built a new one since the early 1990s."

The National Nuclear Security Administration has lost a quarter of its workforce since the 1990s, and half of the scientists at US nuclear labs are over 50 years old, he said.