BEIJING—China has been bolstering defenses along its 880-mile frontier with North Korea and realigning forces in surrounding regions to prepare for a potential crisis across their border, including the possibility of a U.S. military strike.

A review of official military and government websites and interviews with experts who have studied the preparations show that Beijing has implemented many of the changes in recent months after initiating them last year.

They coincide with repeated warnings by U.S. President Donald Trump that he is weighing military action to halt North Korea’s nuclear weapons program, while exerting pressure on China to do more to rein in Pyongyang.

Recent Chinese measures include establishing a new border defense brigade, 24-hour video surveillance of the mountainous frontier backed by aerial drones, and bunkers to protect against nuclear and chemical blasts, according to the websites.

China’s military has also merged, moved and modernized other units in border regions and released details of recent drills there with special forces, airborne troops and other units that experts say could be sent into North Korea in a crisis. They include a live-fire drill in June by helicopter gunships and one in July by an armored infantry unit recently transferred from eastern China and equipped with new weaponry.


China’s Defense Ministry didn’t respond directly when asked if the recent changes were connected to North Korea, saying only in a written statement that its forces “maintain a normal state of combat readiness and training” on the border. It has denied previous reports of thousands of extra Chinese troops moving into border areas.

A Chinese Foreign Ministry spokesman on Monday said: “Military means shouldn’t be an option to solve the Korean Peninsula issue.”

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Chinese authorities have nonetheless been preparing for North Korean contingencies, including economic collapse, nuclear contamination, or military conflict, according to U.S. and Chinese experts who have studied Beijing’s planning.

China’s recent changes in force structure, equipment and training are connected to nationwide military reforms launched last year to overhaul Soviet-modeled command structures and prepare better for combat beyond China’s borders, those experts say.


In the northeast, however, those reforms are geared predominantly toward handling a North Korean crisis, the experts say.

China’s contingency preparations “go well beyond just seizing a buffer zone in the North and border security,” said Mark Cozad, a former senior U.S. defense intelligence official for East Asia, now at the Rand Corp.

“Once you start talking about efforts from outside powers, in particular the United States and South Korea, to stabilize the North, to seize nuclear weapons or WMD, in those cases then I think you’re starting to look at a much more robust Chinese response,” he said. “If you’re going to make me place bets on where I think the U.S. and China would first get into a conflict, it’s not Taiwan, the South China Sea or the East China Sea: I think it’s the Korean Peninsula.”

China, like many foreign governments, still considers a U.S. military strike unlikely, mainly because of the risk of Pyongyang retaliating against South Korea, an American ally whose capital of Seoul lies within easy reach of the North’s artillery.


The Pentagon declined to discuss U.S. planning efforts. American officials didn’t respond to questions about steps taken by China. But top American officials say they are focused on diplomatic and economic pressure, and view military action as a last resort.

Although technically allied to Pyongyang, Beijing wouldn’t necessarily defend its regime, but is determined to prevent a flood of North Koreans from entering northeastern China and to protect the population there, U.S. and Chinese experts say.

Beijing also appears to be enhancing its capability to seize North Korean nuclear sites and occupy a swath of the country’s northern territory if U.S. or South Korean forces start to advance toward the Chinese border, according to those people.

That, they say, would require a much larger Chinese operation than just sealing the border, with special forces and airborne troops likely entering first to secure nuclear sites, followed by armored ground forces with air cover, pushing deep into North Korea.


It could also bring Chinese and U.S. forces face to face on the peninsula for the first time since the war there ended in 1953 with an armistice—an added complication for the Trump administration as it weighs options for dealing with North Korea.

Beijing has rebuffed repeated American requests to discuss contingency planning, American officials say.

China has long worried that economic collapse in North Korea could cause a refugee crisis, bring U.S. forces to its borders, and create a united, democratic and pro-American Korea. But China’s fears of a U.S. military intervention have risen since January as Pyongyang has test-fired several missiles, including one capable of reaching Alaska.

“Time is running out,” said retired Maj. Gen. Wang Haiyun, a former military attaché to Moscow now attached to several Chinese think tanks. “We can’t let the flames of war burn into China.”

He wrote an unusually outspoken article for one of those think tanks in May arguing that China should “draw a red line” for the U.S.: If it attacked North Korea without Chinese approval, Beijing would have to intervene militarily.

A man in Dandong offered tourists the use of binoculars to view the North Korean side of the Yalu River in April. Photo: Damir Sagolj/REUTERS

China should demand that any U.S. military attack result in no nuclear contamination, no U.S. occupation of areas north of the current “demarcation line” between North and South, and no regime hostile to China established in the North, his article said.

“If war breaks out, China should without hesitation occupy northern parts of North Korea, take control of North Korean nuclear facilities, and demarcate safe areas to stop a wave of refugees and disbanded soldiers entering China’s northeast,” it said.

Maj. Gen. Wang said he didn’t speak for the government. But his article isn’t censored online—as it would likely be if Beijing disapproved—in China and other Chinese scholars and military figures recently voiced similar views.

In recent weeks, some details of China’s preparations have also emerged on the military and government websites.

The new border defense brigade patrolled the entire frontier in June to gather intelligence and has drawn up detailed plans for sealing it in a crisis, according to the military’s official newspaper.

Aerial drones would help identify targets, supplementing the new 24-hour video surveillance and addressing problems with “information access, rapid mobility and command and control,” another report in the newspaper said.

Many other units in the northeast have recently conducted new combat-focused training for the kind of joint military operations that experts say would be needed for an intervention within North Korea.

In one drill, a new “combined arms brigade” simulated battle against a “blue team” with artillery, tanks and helicopters, state television reported in June.

The new Northern Theater Command, which controls forces in the northeast, also now incorporates units in eastern China that experts say could be launched across the Yellow Sea toward North Korea.

Meanwhile, authorities in Jilin province, which borders North Korea, are reinforcing and expanding a network of underground shelters and command posts to withstand air, nuclear or chemical attack, local government notices show.

Chinese soldiers recently patrolled the streets of Dandong near the North Korean border. Photo: Jiwei Han/ZUMA PRESS

Such facilities were needed “to respond to the complicated security situation surrounding the province,” Jilin’s civil air defense bureau said in a notice on its website, which also features photos and specifications of U.S. military aircraft.

In May, Jilin’s government unveiled what it called China’s first “combat-ready big data disaster preparedness center” in an underground facility designed to protect critical military and government data from nuclear or chemical attack.

Jilin authorities declined to comment, citing the sensitivity of the subject.

China’s military reforms aren’t complete and the People’s Liberation Army, or PLA, remains ill-prepared for a North Korean operation, some experts say.

“I don’t see the PLA at this time being particularly enthusiastic about being tasked to undertake a potential near-term mission in North Korea,” said Dennis Blasko, a former U.S. military attaché in Beijing.

But China, like the U.S., has been surprised by how fast North Korea’s nuclear weapons program has progressed, say foreign diplomats and experts. Beijing also worries that Pyongyang’s actions are now harming Chinese security interests, since the U.S. deployment in South Korea in April of a missile-defense system that China says can track its own nuclear missiles, diplomats and experts say.

Beijing’s interests “now clearly extend beyond the refugee issue” to encompass nuclear safety and the peninsula’s long-term future, said Oriana Skylar Mastro, an assistant professor at Georgetown University who has studied China’s planning for a North Korean crisis.

“China’s leaders need to make sure that whatever happens with (North Korea), the result supports China’s regional power aspirations and does not help the United States extend or prolong its influence,” Ms. Mastro said.

—Ben Kesling in Washington contributed to this article.

Write to Jeremy Page at jeremy.page@wsj.com