The escalation in the war of words comes after President Donald Trump tweeted Kim Jong Un "won't be around much longer."
Securing North Korea's missile launchers and nuclear, chemical and biological weapons sites would likely be a chief priority for China in the event of a major crisis involving its communist neighbour, analysts say, although Beijing so far is keeping mum on any plans.
Despite China's official silence, its People's Liberation Army likely has a "vast array" of contingency plans involving military options, said Dean Cheng, an Asia security expert at the Heritage Foundation think tank in Washington.
The PLA and paramilitary People's Armed Police could also be deployed to deal with refugees and possible civil unrest, he said.
In this August 29, 2017 photo, North Korean soldiers prepare fencing on the North Korea side of the border with China near the Chinese city of Jian. Photo / AP
What's less clear is whether and under what conditions China would commit troops as an occupying force should North Korean leader Kim Jong Un's regime fall apart, Cheng said.
"We can hypothesise that they might, but, as the observation goes, those who know don't say and those who say probably don't know," he said.
With tensions between the US and North Korea running high and relations between Beijing and Pyongyang at a historic low, questions are being raised about how China might respond in the event of a regime collapse.
The scene along the China-North Korea border in the wild mountains of northeast Asia provides some clues.
Despite a dearth of traffic and trade, construction crews are at work on a six-lane highway to the border outside the small Chinese city of Ji'an along the Tumen River, a corridor that could facilitate the rapid movement of tanks and troops.
Guard posts, barbed wire-topped fences and checkpoints manned by armed paramilitary troops mark the frontier along the border - signs of concern about potentially violent border crossers or even more serious security threats.
China's unwillingness to discuss its plans is likely a strategic choice by the notoriously secretive PLA, but potentially threatens unintended consequences were a major crisis to emerge, experts say.
A board warns against harbouring people who cross the border is displayed near Lingjiang on the border with North Korea in northeastern China's Jilin province. Photo / AP
A man takes selfie at the border which separates China, North Korea and Russia, in Yanbian. Photo / AP
"Each party has its own plans for action in the event of an emergency, but if they act individually without communicating with others, it raises the possibility of misjudgment and unnecessary military conflicts," said Jia Qingguo, dean of the school of International Studies at elite Peking University.
"There has long been a danger in this respect. Someone must take control of North Korea's nuclear weapons," Jia said.
Coordination is also needed on the handling of civilians, particularly with those international agencies experienced in dealing with such crises, Jia said. Among the refugees may be tens of thousands released from North Korean labor camps who may need medical treatment for communicable diseases and malnutrition.
"Refugees are a huge issue that could involve a tremendously large number of people and potentially become a humanitarian crisis," Jia said.
Asked about Chinese preparations for a North Korean crisis, defense ministry spokesman Col. Wu Qian offered assurance but no details at a monthly news briefing on Thursday.
"Dialogue and consultation is the only effective way to solve the problem concerning the Korean Peninsula, and the military option cannot be an option," Wu said.
Chinese tourists stand watch on Tumen bridge linking China and North Korea. Photo / AP
"The Chinese military has made all necessary preparations to safeguard national sovereignty and security and regional peace and stability."
US officials say the Chinese have been reluctant to discuss planning for a major crisis, possibly to avoid offending Kim's notoriously tetchy regime.
Partly in hopes of facilitating such discussions, the two sides signed an agreement during a visit to Beijing in August by the top US military officer to establish a dialogue mechanism between their militaries.
Tellingly, the visit by General Joseph Dunford, the chairman of the Joint Chiefs of Staff, also included the rare opportunity to observe a Chinese army drill near the Chinese city of Shenyang, roughly 200km from the border with North Korea.
Although Dunford said China didn't appear ready to have conversations about responding to a North Korean crisis, senior administration officials say the sides recognise the need for communication on the matter and the topic has been broached in semiofficial talks between experts.
While Chinese officials have routinely said Beijing would not allow "chaos and war" to break out on its doorstep, official media have hinted that it might not respond if the North made an unprovoked strike on the U.S. or its allies and suffered a retaliation.
Chinese custom officials inspect trucks loaded with goods to and from North Korea in Dandong in northeast China's Liaoning province. Photo / AP
That ambiguity serves to keep the US and South Korea guessing, possibly tempering their own responses, said Cheng, the Asia security expert.
Beijing also doesn't want to publicise any plans to avoid provoking Pyongyang, either by revealing doubts about the stability of Kim's regime or by exposing its deepest worries that Kim could then leverage for his own benefit, Cheng said.
"Positing that they have so little confidence that they are planning for (Pyongyang's) demise might create the very problem that they fear," he said.
"Better the devil that you know."
With 85 percent of North Korea's nuclear facilities located within 100km of the border with China, special forces from the People's Liberation Army could easily secure those sites without coming into conflict with occupying forces from South Korea and the US, said Georgetown University security studies professor Oriana Skylar Mastro.
Workers watch a machine work at a construction site across the Tumen river, the border river between North Korea and China, in Yanbian. Photo / AP
PLA forces might also cross the border to carry out missions to stabilize refugees, Mastro said. The Chinese military's ability to deal with such a contingency has been honed over recent years through its participation in United Nations peacekeeping missions in Africa and elsewhere, as well as its leading role in responding to earthquakes, floods and other disasters within China.
In the long term, Beijing would want to see a friendly government in Pyongyang to ease security concerns about a unified Korea under the protection of US and South Korean troops, against whom China fought in the 1950-53 Korean War. China has long criticized American military alliances in Asia, seeing them as part of a campaign to stifle its rise as Asia's leading power.
Underlying questions about a Chinese crisis response is the dismal state of China-North Korea relations, illustrated by the lack of regular high-level exchanges. Xi Jinping is the first Chinese leader to visit South Korea before traveling to the North, which he has yet to do as president. Xi and Kim, who has not traveled to China as leader, are not known to be in direct contact, and no senior Chinese official has visited North Korea in almost two years.
Also rarely seen until recently are complaints about China in the North Korean official media, prompted by Beijing's support for UN sanctions on Pyongyang.A recent commentary by the North's official Korean Central News Agency said China's ruling Communist Party's mouthpieces were "going under the armpit of the US" by criticising Pyongyang's weapons programme.
It accused party news outlets of "kowtowing to the ignorant acts of the Trump administration."
