Whatever contingencies exist to handle some sort of serious breakdown in a nuclear-armed North Korea are not shared by China, the United States or Washington's treaty allies Japan and South Korea.

The more alarming scenarios for the collapse of the world's last Stalinist state - and one of its poorest - are not difficult to imagine.

One of the worst discussed by diplomats in Washington, Tokyo and Seoul has Chinese and US-backed South Korean troops squaring off inside a crumbling North, both attempting to find and secure nuclear facilities. Another has Chinese troops sealing China's border not only against a tide of refugees heading for its still-poor northeast but in order to seize and hold territory as a potential buffer zone against the prospect of a unified Korea led by South Korea.

"Would South Koreans view the PLA as an invasion force in their country? Would the US stay back inside North Korea? Would the US seek to use its Japanese-based forces?" pondered one security scholar and Japanese government adviser. "The more one looks at the scenarios, the more questions there are … and the more we all need to talk, preferably on a very quiet level."

Already US and South Korean forces have staged war games based on an urgent need to lock down facilities inside North Korea.

Just a few years ago, the so-called six-party talks geared to denuclearising North Korea raised hopes of a new era in long-term dialogue, raising trust among China, the US, South Korea, Japan and Russia as they worked with Pyongyang.

Such was the optimism that even then-South Korean foreign minister Ban Ki-moon raised the prospect of using the six-party concept as a long-term model for a new regional security body as he lobbied for his current job - secretary general of the United Nations. The six-party talks have, of course, repeatedly failed to end Pyongyang's nuclear recalcitrance and the wider picture in Northeast Asia remains as fraught as ever.

"In the beginning, there was a certain optimism that somehow the six-party talks would, over time, see us sit around with Chinese counterparts and hear about their views on North Korean intentions and how we could all deal with a collapse," said one senior Asian diplomat close to the talks.

"It never even started to happen," he said. "Beyond the official agenda items, anything else was apparently deemed too sensitive. If we pushed into areas like contingency planning, the walls came down.

"For the Chinese side, there was the six-party talks on one side, and then there was their own deep and complex relationship with North Korea to consider."

Back-channel discussions on scenarios involving leading mainland and American scholars staged in both Beijing and Washington have never progressed very far, including some held quietly within the last year, say participants on both sides.

Bonnie Glaser, a scholar with the independent Centre for Strategic and International Studies in Washington, said she had frequently raised the need for better dialogue over North Korea with many Chinese contacts, including high-level officials, but it was clear that Beijing was wary of discussing North Korea's possible collapse.

"In their judgment, there are no signs that there is pending instability in North Korea," she said.

"They don't see the urgency."

While noting that talks might be possible if the situation worsened, the Chinese also had suspicions that the US and South Korea, by seeking such discussions, would try to gain an insight into Chinese plans.

"The Chinese are worried that North Korea would find out if they are talking about contingencies with the US and South Korea, and would strongly object and possibly take retaliatory measures.

"As for talking with Japan, the Chinese don't see a role for Japan in any contingency on the Korean Peninsula."

Preparatory talks among key players would not only lower suspicion and allow for a clear division of labour in securing nuclear weapons, but would also help co-ordinate plans for international humanitarian aid to North Koreans.

"If US and [South Korean] forces entered the North, the potential for China's direct involvement increases greatly," Glaser said. "In the absence of prior co-ordination among the US, South Korea and China, forces on the ground could come into conflict."

Professor Zha Daojiong, an international relations scholar at Peking University, acknowledged previous Chinese sensitivities over contingency discussions involving complex questions such as regime change in North Korea or the prospect of refugee camps in China.

But he noted that the latest crisis might have opened up a fresh avenue - fears about nuclear safety.

He said reports of mounting concern among Chinese officials and scholars about their North Korean ally were accurate.

They ranged from the direction of North Korea's leadership to worries about the impact on the military policies and behaviour of both Japan and South Korea as the nuclear arsenal of Pyongyang grew in size and sophistication.

As testing intensified, the risk of some kind of nuclear accident affecting China could also no longer be disregarded, he said. This posed a clear domestic political challenge to the leadership in Beijing, he said.

Zha said that, while once such societal concerns would have been kept under the rug, "I believe we have not paid due concern to that … in this microblogging age, it is not hard to imagine public fears over radiation quickly becoming a domestic governance challenge as people swap information about the situation on the ground.

"If there is room to start discussion with the US and other countries, it is possibly in this area of nuclear safety, rather than contingencies involving a rather broad and uncertain agenda."

Amid the questions, Zha said one certainty should not be overlooked - that North Korea had no interest in giving up its weapons programme.

"It is hard to imagine North Korea simply giving up its nuclear ambitions - they look around the world and see it as their right.

"Many times the North Koreans have said to us, 'Why are you so concerned? We are only doing what Chairman Mao [Zedong] did in the 1960s'.

"It is very hard to reason with them."