Scientists and officials say the test almost certainly did not involve a hydrogen bomb. But the fourth test of any kind of nuclear device by the isolated country would signal its continuing defiance of the outside world - including China, which has long expressed displeasure with Pyongyang's nuclear programme.
Experts say Beijing's influence over Pyongyang has diminished since Kim Jong Un took over as North Korea's leader at the end of 2011 and Xi Jinping became President of China in 2013.
The two men have not met since assuming power, with Xi even snubbing his counterpart by visiting South Korea first in 2014.
In October, there was talk of a thaw when Xi sent an envoy to attend a military parade in Pyongyang with a signed letter carrying the Chinese President's "best wishes" to Kim. But relations swiftly turned frigid again in December, after Kim declared that his country had developed a hydrogen bomb.
Xuan Dongri, director of Northeast Asia Studies at Yanbian University, said: "China and North Korea's understanding of each other is deteriorating further."
Bo said a key problem has been the centralisation of power in Beijing under Xi. Instead of the multifaceted policy towards North Korea that prevailed under Xi's predecessor, Hu Jintao, with some senior leaders advocating engagement and others taking a harder line, Xi is calling all the shots.
Since Xi has not met Kim and has his hands full with policy challenges, there is little meaningful dialogue taking place and little internal debate on how best to influence Pyongyang. "You need to have a connection if you want to convince or persuade the other side. If you don't have a connection, where is the leverage?"
Bo said Xi was "caught in a dilemma", reluctant to hew closer to the US approach of isolating and punishing North Korea but "powerless" to prevent North Korea's nuclear programme.
Analysts are not predicting a complete breakdown in Sino-North Korean ties. Nor do they foresee North Korea abandoning the nuclear programme that has become a key pillar of its regime's declared legitimacy, at least not soon.
"Beijing will face increased pressure both domestically and internationally to punish and rein in Kim Jong Un," said Yanmei Xie, senior China analyst with the International Crisis Group in Beijing. "But there is likely to be a repeat of the worn playbook of denunciation, tightening of sanctions and calling for resurrection of the six-party talks."
China is North Korea's largest trading partner. It has been unwilling to pull the plug for fear of toppling the Kim regime.
Paul Haenle of the Carnegie-Tsinghua Centre in Beijing said China is likely to move cautiously, but he did not rule out a tougher line.
"North Korea's defiance is not only an untenable burden on China's image as a credible and strong leader on this issue, but will also lead to an enhanced US security posture in the region and increased cooperation between the US and its Asia-Pacific allies - not something Beijing wants."
"While analysts have plenty of evidence to justify their assessments that China won't change course, I think we need to be open to the possibility that China could respond differently this time."
How to tell if it's a nuke:
Seismology: There are hundreds of sensors around the world designed to pick up the seismic activity associated with nuclear weapons. Nukes set off underground create vibration patterns that can be easily distinguished from earthquakes, drilling or other activity. Sensors are good at pinpointing the location of a nuclear test.
Atmospheric sampling: One of the telltale signs is radiation. If the testers are trying to cover their tracks, there may not be large pieces of radioactive evidence. But there may be gases that can seep through earth and rock. So watchdogs will "sniff" the atmosphere for xenon, which doesn't bind easily to other elements and makes it easier to link to a nuclear test. By analysing wind patterns, experts can find the source of the xenon.
Acoustics: There's a network of underwater microphones to listen for nuclear-related vibrations. Sound travels well underwater: experts can identify violent disruptions against ordinary ocean noise.
What we know:North Korea claims to have blown up an H-bomb; scientists detected a 4.9-magnitude seismic event; Japan and the US are sending jets to sniff for radioactive isotopes. Investigators may not find any xenon gases to speak of, as was the case after North Korea's test in 2009. In 2013, researchers could only pick up inconclusive amounts.
