BEIJING (NYTIMES) - As a presidential candidate, Donald Trump talked about China almost solely in the context of trade, such as when he promised to slap punishing tariffs on cheap Chinese imports that he argued had ruined the lives of working-class Americans.

But as he begins to feel the weight of office on his shoulders, there are signs that his focus in the Asia region may have shifted toward security - specifically, the problem of North Korea and its expanding nuclear arsenal, which experts say already threatens the United States' regional allies, Japan and South Korea.

During his visit to The New York Times this week, Trump referred obliquely to a "big problem for the country" that President Barack Obama had mentioned during their 90-minute meeting at the White House after the election. Well-placed US officials believe that reference was to North Korea.

Any solution to that problem must involve China, North Korea's patron, US and Chinese officials agree. Trump acknowledged as much during the campaign, saying on one occasion that China should do more to bear down on the North.

So which tack will Trump take with China? Will he seek its support for a deal on North Korea, or will he start a trade war, putting such cooperation in doubt?

US officials briefed the Chinese several months ago on their assessment that the North's nuclear capabilities had sharply increased, according to two Americans with knowledge of the briefings, who spoke on the condition of anonymity because of the sensitivity of the matter.

But even before then, nuclear scientists from China and the US had agreed for some time about the danger posed by the North's ambitions, said Siegfried S. Hecker, a US nuclear scientist at Stanford University who was the last outsider to visit the country's plutonium processing plant at Yongbyon, in November 2010.

Several years ago, the Chinese tended to play down the North's nuclear capabilities, Hecker said in an email. But since the mercurial Kim Jong Un, a young leader who has declined to listen to China and has provoked the US, took over the country, the assessments of the two powers have been "pretty much in line," he said.

Hecker and other scientists estimate that North Korea might develop the capacity to strike the West Coast of the US with a nuclear warhead in about five years. But the real problem is here and now, in Asia, he said.

"The greatest and most urgent threat comes not from a North Korean nuke being able to reach the US, but rather what they have already," he said. Specifically, Hecker wrote in a recent article, the North is now probably able "to put nuclear weapons on target anywhere in South Korea and Japan and even on some US assets in the Pacific."

Worse yet, he wrote, North Korea may have developed a "false sense of confidence" from a recent spate of successful nuclear and missile tests - one that could lead it to grave errors of judgment.

By the end of this year, Hecker estimated, the country is likely to have enough fissile material for about 20 bombs. The danger would be exacerbated if the North decided "to field tactical nuclear weapons as its arsenal expands and its confidence in its nuclear arsenal grows." In short, Hecker said, more and better bombs make a catastrophic miscalculation by North Korea more likely.

How much of this Trump knew before his victory is unclear, but he was certainly aware of the problem. Early on, he expressed a mixture of awe and dismay toward Kim.

"If you look at North Korea, this guy, I mean, he's like a maniac, OK?" he said at a rally in Ottumwa, Iowa, in January. "And you've got to give him credit. How many young guys - he was like 26 or 25 when his father died - can take over these tough generals?" Trump was referring to Kim's execution of several generals, including his uncle, Jang Song Thaek, who was the main conduit between North Korea and China.

"We can't play games with him," Trump said at the time, referring to Kim. "Because he really does have missiles, and he really does have nukes."

Such considerations - and briefings that Trump presumably has received or will receive on the North's capabilities - could compel the new president to prioritise security over trade in his dealings with China.

With the right approach, he could find a willing partner in Beijing, said Yang Xiyu, a former Chinese official who oversaw the so-called six-party talks on the North's nuclear program that collapsed in 2008.

But Chinese officials say that approach would require removing a thorn in Beijing's side: an advanced missile defence system that the US plans to install in South Korea. The Chinese view that system, known as the Terminal High Altitude Area Defence System, as an effort to contain its ambitions in Asia, though Washington and Seoul say it is intended purely to defend South Korea against the North.

In the past year, China has rebuffed many of Washington's requests for sharper measures against North Korea, and the missile system is one reason. Whether Trump would consider scrapping it is another unknown.

Washington and Beijing did cooperate on United Nations sanctions against the North this year - an encouraging sign, analysts said, though many doubt that economic punishment alone will be a deterrent.

And on Friday, the Chinese state-run news media reported that Beijing would support a new round of sanctions being considered by the Security Council that would close loopholes allowing North Korea to sell coal.

Yang, the former official, said the Trump administration must take into account Beijing's worst fear, where the North is concerned: a collapse of Kim's totalitarian regime, followed by refugees from the North pouring into China and a unified Korean Peninsula under the protection of the US. military. As president, Trump must understand that Kim's regime has to stay, Yang said.

"Remove the bombs, not the regime, is the key to a peaceful solution," Yang said. If the US shares that goal, he said, "we can cooperate sincerely and substantively."