WASHINGTON (NYTIMES) - When negotiators from the United States and Russia met in Vienna last week to discuss renewing the last major nuclear arms control treaty that still exists between the two countries, US officials surprised their counterparts with a classified briefing on new and threatening nuclear capabilities - not Russia's, but China's.

The intelligence had not yet been made public in the US, or even shared widely with Congress. But it was part of an effort to get the Russians on board with President Donald Trump's determination to prod China to participate in New START, a treaty it has never joined.

Along the way, the administration is portraying the small but increasingly potent Chinese nuclear arsenal - still only one-fifth the size of those fielded by the US or Russia - as the new threat that Mr Trump and President Vladimir Putin of Russia should confront together.

Mr Marshall Billingslea, Mr Trump's new arms control negotiator, opened his classified briefing, officials said, by describing the Chinese programme as a "crash nuclear build-up", a "highly alarming effort" to gain parity with the far larger arsenals that Russia and the US have kept for decades.

The American message was clear: Mr Trump will not renew any major arms control treaty that China does not also join - dangling the possibility that Mr Trump would abandon New START altogether if he did not get his way. The treaty expires in February, just weeks after the next presidential inauguration.

Many outside experts question whether China's build-up - assessed as bringing greater capability more than greater numbers - is as fast, or as threatening, as the Trump administration insists.

The intelligence on Beijing's efforts remains classified, a senior administration official said, noting that sharing such data is not unusual among the world's major nuclear weapons states. But that means it was given to an adversary with whom the US is conducting daily, low-level conflict - including cyber attacks, military probes by warplanes and Russian aggression in Ukraine. And that was before reports surfaced that a Russian military intelligence unit had put bounties on US and allied troops in Afghanistan.

The US official said the administration would try to declassify and make public some of the assessment about China.

Nuclear weapons have suddenly become a new area of contention between Mr Trump and President Xi Jinping of China, and there are many reasons to believe that even if the three superpowers are not yet in a full-scale arms race, what is taking place in negotiating rooms around the world may soon start one.

The Russians have publicly offered a straight, five-year extension of New START, which would not require congressional approval. But Mr Trump is clearly betting that he can find common ground with Mr Putin in confronting the Chinese.

Without question, the Chinese are improving their arsenal, and may be rethinking the idea of holding a "minimal deterrent" - just enough to assure that if they were ever attacked they could take out cities in Russia, Europe or the US.

But they have only 300 long-range nuclear weapons deployed, compared with 1,550 each that the other two superpowers are allowed under New START. So there is the very real possibility, experts say, that in any negotiation, Beijing will insist on quintupling its nuclear force before it agrees to any constraints.

So far, China has said it is not interested in discussing any limitations.

"The notion of trying to pull the Chinese into that agreement is, in theory, a good idea. In practice? Impossible," former defence secretary Robert M. Gates said this month at the Centre for Strategic and International Studies.

"The Chinese have no incentive whatsoever to participate," said Mr Gates, who as CIA director confronted China over its sale to Iran of missiles designed to carry nuclear warheads. And if Mr Trump continues on the current course, Mr Gates said, he will end up essentially inviting "the Chinese to build dramatically more, far more, nuclear weapons than we think they have at the current time to get level with the United States".

Nuclear weapons are joining the panoply of issues - including trade deals, banning Chinese students and wiring the world for 5G networks - that Mr Trump has put at the centre of a series of US-China standoffs.

Mr Trump is no student of nuclear history, but in some ways he is replaying a moment from the 1960s, when late Chinese leader Mao Zedong was seeking nuclear weapons.

President Lyndon B. Johnson's administration briefly considered inviting the Soviets to participate in a joint strike at Lop Nor, the Chinese nuclear testing site, to prevent the country from joining the nuclear club.

But the Americans abandoned the idea, determining it was simply too dangerous. A top secret State Department study, since declassified, concluded in April 1964 that the risk of a Chinese nuclear capability "is not such as to justify the undertaking of actions which would involve great political costs or high military risks".

The US has lived with a Chinese "minimal deterrent" for 56 years.

Now Mr Billingslea argues that new activities under way at Lop Nor, combined with China's far greater reach in space and at sea, once again put America at risk. The Chinese, not surprisingly, blame the US, saying the American focus on missile defences is forcing them to develop a counterforce of new nuclear weapons and missiles.

"If Beijing's concerns are left unaddressed, they will likely fuel more intensive Chinese efforts to modernise its nuclear forces and other strategic capabilities," Dr Tong Zhao , a senior fellow at the Carnegie-Tsinghua Centre for Global Policy in Beijing, wrote recently.

The roots of the revival of interest in building up nuclear arsenals go back to the passage of New START a decade ago, early in the Obama administration.

As the price of getting the treaty through the Senate, President Barack Obama agreed to a multibillion-dollar upgrade of the American nuclear complex, including production facilities that had been neglected for decades. At the same time, vice president Joe Biden, now Mr Trump's presumed opponent in the presidential election, said the administration would ask the Senate to ratify the Comprehensive Nuclear Test Ban Treaty, which former president Bill Clinton had signed but the Senate had never acted on.

Mr Obama and Mr Biden never sought ratification, realising they would lose. But the past four presidents have abided by the treaty's ban on nuclear tests. That may be coming to an end: Mr Billingslea confirmed that the Trump administration had discussed "unsigning" the treaty and debated whether the US should return to nuclear testing, which it has not engaged in since 1992. But he said there was no need to do so for now.

The US conducted more nuclear tests during the Cold War than the rest of the world combined. Over decades of experimentation, and more than 1,000 tests, its bomb designers learned many tricks of extreme miniaturisation as well as how to endow their creations with colossal destructive force. Compared with the atomic bomb that levelled Hiroshima, Japan, the nation's first explosive test of a hydrogen bomb, in 1954, produced a blast 1,000 times as powerful.

Because of that history, many nuclear experts now argue that if Mr Trump begins a new wave of global testing, it would aid American rivals more than the US.

"We lose more than we gain," Dr Siegfried S. Hecker, a former director of the Los Alamos weapons laboratory in New Mexico and now a professor at Stanford University, said in an interview.

Beijing had conducted only 45 tests, he noted, and would welcome a resumption of testing to "increase the sophistication or perhaps the diversification" of its arsenal, "and that can only come back to be a national security risk for the United States".

Activity at the desert testing site in Nevada has soared in recent years. There is new drilling, construction, equipment, employees and periodic "subcritical" tests, just below the threshold of producing a nuclear explosion.

For years, some Republicans have urged preparations for a test and poured money into the effort. One instrument now being prepared for the Nevada complex costs US$800 million (S$1.1 billion); it would test the behaviour of plutonium.

Today, Republicans are still urging more upgrades and speedups, including at the Nevada complex. This month, Republican senator Tom Cotton, offered an amendment to a defence Bill that would add at least US$10 million to "carry out projects related to reducing the time required to execute a nuclear test".

Top Democrats in the House told the Pentagon and the Energy Department in a recent letter that the idea of a renewal in nuclear testing was "unfathomable", as well as "shortsighted and dangerous".

But Mr Billingslea thinks he succeeded in getting the Russians to think about what is happening in China, not in the Nevada desert. During his meeting last week, the Russians were taking copious notes on China's buildup, while reviewing classified slides. He insists they want to sit down and talk more later in the summer.

They will do so without the Chinese.