SEOUL (WASHINGTON POST) - In discussions between the United States and China about reining in North Korea, one topic remains taboo: What would happen if North Korea leader Kim Jong Un's regime collapses?

For years, China has rebuffed US attempts to raise the topic at so-called Track 2 dialogue sessions between academics in each country's foreign policy establishment, according to senior adviser for Asia Bonnie Glaser at the Centre for Strategic and International Studies (CSIS) in Washington, who has led the US side in such talks.

Attendees included people from Chinese government-affiliated research institutions and military officers, she said.

From China's perspective, officially broaching the issue could alarm its neighbour, which has received Beijing's backing since the Korean War in the 1950s. There is also a fear that it would give the US an advantage in one day reunifying the Korean peninsula on its terms.

"When we first started these efforts, the Chinese told us - and many other people, including US officials - that if they engaged in such discussions with the United States it would come out, it would get leaked, North Korea would find out and they would retaliate," Ms Glaser said.

Still, as the US and North Korea trade threats of military action and Mr Kim - who is believed to be in his early 30s - develops nuclear weapons, observers are starting to game out worst-case scenarios.

President Donald Trump told reporters on Tuesday (Sept 26) that the US is prepared to use "devastating" military force against North Korea, though it is not a preferred option.

No matter whether a military miscalculation, coup or other event prompts the demise of Mr Kim, it may not be long before soldiers from the world's two biggest economies come face-to-face.

The US has about 28,500 troops stationed in South Korea, which has another 625,000 in its standing army and 3.1 million in reserve. China has roughly 2 million soldiers, and its Northern Theatre Command bordering North Korea consists of at least three army units and three rocket units.

"If we intervene and the Chinese run into our people and if we run into their people, what are we going to do?" defence researcher Bruce Bennett at Rand Corp, who wrote a 342-page report in 2013 on policy recommendations in case North Korea collapses, said by phone. "Are we going to shake hands or what? All of that needs to be really thought about seriously."

If North Korea starts to collapse, both the US and China are likely to send in troops to secure its weapons of mass destruction, Mr Bennett said. Most of those sites - including the Yongbyon nuclear plant - are closer to China than South Korea.

US intelligence officials have concluded that North Korea can miniaturise warheads to fit on missiles, and has as many as 60 nuclear bombs, The Washington Post reported last month.

South Korea's 2016 Defence White Paper said Pyongyang also has a stockpile of between 2,500 tons and 5,000 tons of chemical weapons, and is capable of producing biological agents such as anthrax and smallpox.

Any incursion by Chinese troops into North Korea risks drawing in South Korea, whose constitution covers the entire peninsula. Similar to China, authorities in Seoul would also be worried about large flows of refugees who might flee, particularly if war leads to shortages of food and other essential goods.

Professor Jia Qingguo, dean of the School of International Studies at Peking University, wrote in a Sept 11 article for the East Asia Forum that China's military should consider creating a safety zone within North Korea to prevent a large flow of people into its northeastern provinces.

Authorities in Beijing would probably also want to discuss overseeing any situation that requiring the US to move north of the 38th parallel, including securing North Korea's nuclear weapons and restoring order in Pyongyang, Prof Jia wrote.

Another issue will be whether to let the international community oversee the formation of a new government in North Korea.

"Beijing doesn't have a good plan, the US doesn't have a good plan, and the entire world doesn't have a good plan," Prof Jia said by phone. "It's beyond the point that a neat solution can be found. So, hope for the best and prepare for the worst."

China certainly has planned for worst-case scenarios including refugees and potential nuclear proliferation, said international relations professor Su Hao at China Foreign Affairs University, which is affiliated with the country's Foreign Ministry.

"It's strategically premature for China to discuss it with South Korea or the US," Prof Su said. "However, it's likely for China to inform Russia because their interests on the peninsula are more aligned."

In public, all the major powers say they agree on the goal of a denuclearised Korean peninsula. The US, China and Russia have also said they back the so-called Four Nos: No regime change, regime collapse, accelerated reunification and military deployment north of the 38th parallel.

Still, mistrust remains. Mr Trump's recent comments to the United Nations that communist and socialist governments are failed ideologies that contribute to human suffering may fuel suspicions within China that the US ultimately seeks regime change in Beijing, according to Ms Glaser at CSIS.

"Let's remember that the Chinese believe that the 'colour revolutions' were really aimed at overthrowing Communism everywhere," she said, referring to a wave of movements in the former Soviet Union and Balkans in the early 2000s.

"Why would China work with the US and tell it about its contingency plans?"