The problem has always been what China will bear in terms of restricting its protégé and neighbor, as well as whether it will cut back fuel shipments and other trade with North Korea. 
 
 Advertisement Continue reading the main story 
 
 “Moving forward, China really holds the key to what extent the actions will be different this time,” said Suzanne DiMaggio, an expert at the Asia Society. 
 
 The signs are hard to read. 
 
 China will almost certainly join the United States in supporting tougher sanctions over Tuesday’s test, accompanied by sterner reprimands from Beijing against its recalcitrant ally in Pyongyang, which ignored Chinese entreaties not to take provocative actions. 
 
 But as impatient as China might be with North Korea, there is little chance that the new Chinese leader, Xi Jinping, will move quickly to change the nation’s long-held policy of propping up the walled-off government that has long served as a buffer against closer intrusion by the United States on the Korean Peninsula. 
 
 Advertisement Continue reading the main story 
 
 The Chinese military, and to a lesser extent the International Liaison Department of the Chinese Communist Party, assert strong influence on China’s Korea policy, and both powerful entities prefer to keep North Korea close at hand, Chinese and American analysts say. 
 
 Photo 
 
 While the People’s Liberation Army is not even able to conduct military exercises with the North Koreans — the government in the North forbids such contact with outsiders — Chinese military strategists adhere to the doctrine that they cannot afford to abandon their ally, no matter how bad its behavior, analysts here say. 
 
 Advertisement Continue reading the main story 
 
 At the same time, the Chinese Communist Party looks upon the North Korean Communist Party — led by Kim Jong-un, the grandson of the nation’s founder — as a fraternal brotherhood. Indeed, relations between the two countries are conducted largely between the two parties rather than between the two foreign ministries, the more normal diplomatic channel. 
 
 In an early sign that Mr. Xi is unlikely to veer from past policy, the state-run news agency, Xinhua, criticized the United States and its allies for essentially forcing the North’s aggression by causing the country to feel insecure. 
 
 But within this basic contour there could be some adjustments by Mr. Xi, according to Zhu Feng, a professor of international relations at Peking University, an advocate of a tougher policy by China against North Korea. 
 
 Advertisement Continue reading the main story 
 
 “One nuclear test will not make China’s new administration decide to ‘abandon North Korea,’ but it will definitely worsen China-North Korea relations,” Professor Zhu wrote in a recent article in The Straits Times of Singapore. “North Korea’s nuclear test will make the new Xi Jinping administration angry, and give China a headache.” 
 
 Advertisement Continue reading the main story 
 
 Mr. Xi, who became head of the Communist Party and military council in November, will ascend to the presidency of the country next month. 
 
 Newsletter Sign Up Continue reading the main story Please verify you're not a robot by clicking the box. Invalid email address. Please re-enter. You must select a newsletter to subscribe to. Sign Up Receive occasional updates and special offers for The New York Times's products and services. Thank you for subscribing. An error has occurred. Please try again later. View all New York Times newsletters. 
 
 To improve China’s strained relationship with the United States, Mr. Xi — who has shown himself to be more nationalistic than his predecessor — could start with getting tougher on North Korea, including at the Security Council. 
 
 The Obama administration excoriated Mr. Xi’s predecessor, Hu Jintao, after North Korea’s second nuclear test in 2009, accusing him of “willful blindness” to that country’s actions. 
 
 Advertisement Continue reading the main story 
 
 “With Hu out of the picture, the administration is intent on determining whether Xi Jinping will prove more attentive to U.S. security concerns,” said Jonathan D. Pollack, director of the John L. Thornton China Center at the Brookings Institution. 
 
 “How Xi chooses to respond will be an important early signal of his foreign policy priorities and whether he is ready to cooperate much more openly and fully with Washington and Seoul than his predecessor,” he said, referring to South Korea. 
 
 Video 
 
 China’s calculations will be crucial to what happens at the Security Council, where the policy has always been to pursue unanimity over toughness; it is considered far better to get all members on board to send a message to North Korea rather than have China abstain or worse, veto. 
 
 Advertisement Continue reading the main story 
 
 In the absence of any real leverage, Washington and its allies are left warning Beijing that if it does not keep North Korea out of the nuclear club, it risks an arms race in its own neighborhood. 
 
 Advertisement Continue reading the main story 
 
 President Obama spoke Tuesday morning, hours after the test, with South Korea’s outgoing president, Lee Myung-bak, in what the White House described as an effort to agree on a common strategy at the Security Council and in the two country’s military posture on the Korean Peninsula, where troops have faced off against the North since the armistice was signed 60 years ago. 
 
 Advertisement Continue reading the main story 
 
 A statement issued by the White House after the conversation included a rare reference to American’s “nuclear umbrella” over South Korea, a reminder to the North that an attack on South Korea would be viewed as an attack on the United States. While that has been true for decades, it is rarely highlighted in American statements. 
 
 “Threatening a missile-capable warhead with a successful third nuclear test gives the United States, South Korea and Japan good reason to step up their regional ballistic missile defense capabilities,” said Siegfried S. Hecker, former director of the Los Alamos National Laboratory and co-director of the Center for International Security and Cooperation at Stanford University. “That should give the Chinese government some pause.” 
 
 Advertisement Continue reading the main story 
 
 The United States faces its own geopolitical calculations. Some experts say it needs to keep up the tough talk, even if it understands that its efforts at the Security Council may not do much to limit the North’s capabilities. Even the strongest sanctions and increasing isolation have not caused the North to back off its nuclear program, which the leadership sees as both a deterrent to possible American aggression, and which gives the impoverished country’s governing elite a success it can show its suffering people. 
 
 Advertisement Continue reading the main story 
 
 Now experts say the North may be simply trying to wait the United States out, hoping it will eventually recognize its program as it did Pakistan’s. But experts say Mr. Obama cannot recognize North Korea as the nuclear state it increasingly is without giving signals to Iran — which the West believes is pursuing a nuclear weapon despite its denials — that it could try the same path. 
 
 As the world’s powers struggle to refine their policies, North Korea continues to make technological advances. A long-range rocket test in December has been judged by outside experts to have been a success after many failures. 
 
 And Tuesday’s test, if it was as large as originally thought, appears to suggest that the nuclear arms program is also moving ahead. (The first test in 2006 was considered a partial failure.) 
 
 Advertisement Continue reading the main story 
 
 “It moves the question of North Korea as a nuclear contender from ‘if’ to ‘when,’ ” said one senior Obama administration official. “The ‘when’ may still be years away, but at least now it is in sight.” ||||| China has signalled its readiness to accept Korean reunification and is privately distancing itself from the North Korean regime, according to leaked US embassy cables that reveal senior Beijing figures regard their official ally as a "spoiled child". 
 
 News of the Chinese shift comes at a crucial juncture after the North's artillery bombardment of a South Korean island last week that killed four people and led both sides to threaten war. China has refused to condemn the North Korean action. But today Beijing appeared to bow to US pressure to help bring about a diplomatic solution, calling for "emergency consultations" and inviting a senior North Korean official to Beijing. 
 
 China is sharply critical of US pressure tactics towards North Korea and wants a resumption of the six-party nuclear disarmament talks. But the Guardian can reveal Beijing's frustration with Pyongyang has grown since its missile and nuclear tests last year, worries about the economic impact of regional instability, and fears that the death of the dictator, Kim Jong-il, could spark a succession struggle. 
 
 China's moves to distance itself from Kim are revealed in the latest tranche of leaked US embassy cables published by the Guardian and four international newspapers. Tonight, the US secretary of state, Hillary Clinton, said the US "deeply regrets" the release of the material by WikiLeaks. They were an "attack on the international community", she said. "It puts people's lives in danger, threatens our national security and undermines efforts to work with other countries to solve shared problems," she told reporters at the state department. 
 
 The leaked North Korea dispatches detail how: 
 
 • South Korea's vice-foreign minister said he was told by two named senior Chinese officials that they believed Korea should be reunified under Seoul's control, and that this view was gaining ground with the leadership in Beijing. 
 
 • China's vice-foreign minister told US officials that Pyongyang was behaving like a "spoiled child" to get Washington's attention in April 2009 by carrying out missile tests. 
 
 • A Chinese ambassador warned that North Korean nuclear activity was "a threat to the whole world's security". 
 
 • Chinese officials assessed that it could cope with an influx of 300,000 North Koreans in the event of serious instability, according to a representative of an international agency, but might need to use the military to seal the border. 
 
 In highly sensitive discussions in February this year, the-then South Korean vice-foreign minister, Chun Yung-woo, told a US ambassador, Kathleen Stephens, that younger generation Chinese Communist party leaders no longer regarded North Korea as a useful or reliable ally and would not risk renewed armed conflict on the peninsula, according to a secret cable to Washington. 
 
 Chun, who has since been appointed national security adviser to South Korea's president, said North Korea had already collapsed economically. 
 
 Political collapse would ensue once Kim Jong-il died, despite the dictator's efforts to obtain Chinese help and to secure the succession for his son, Kim Jong-un. 
 
 "Citing private conversations during previous sessions of the six-party talks , Chun claimed [the two high-level officials] believed Korea should be unified under ROK [South Korea] control," Stephens reported. 
 
 "The two officials, Chun said, were ready to 'face the new reality' that the DPRK [North Korea] now had little value to China as a buffer state – a view that, since North Korea's first nuclear test in 2006, had reportedly gained traction among senior PRC [People's Republic of China] leaders. Chun argued that in the event of a North Korean collapse, China would clearly 'not welcome' any US military presence north of the DMZ [demilitarised zone]. Again citing his conversations with [the officials], Chun said the PRC would be comfortable with a reunified Korea controlled by Seoul and anchored to the US in a 'benign alliance' – as long as Korea was not hostile towards China. Tremendous trade and labour-export opportunities for Chinese companies, Chun said, would also help 'salve' PRC concerns about … a reunified Korea. 
 
 "Chun dismissed the prospect of a possible PRC military intervention in the event of a DPRK collapse, noting that China's strategic economic interests now lie with the United States, Japan and South Korea – not North Korea." 
 
 Chun told Stephens China was unable to persuade Pyongyang to change its self-defeating policies – Beijing had "much less influence than most people believe" – and lacked the will to enforce its views. 
 
 A senior Chinese official, speaking off the record, also said China's influence with the North was frequently overestimated. But Chinese public opinion was increasingly critical of the North's behaviour, the official said, and that was reflected in changed government thinking. 
 
 Previously hidden tensions between Pyongyang and its only ally were also exposed by China's then vice-foreign minister in a meeting in April 2009 with a US embassy official after North Korea blasted a three-stage rocket over Japan into the Pacific. Pyongyang said its purpose was to send a satellite into orbit but the US, South Korea and Japan saw the launch as a test of long-range missile technology. 
 
 Discussing how to tackle the issue with the charge d'affaires at the Beijing embassy, He Yafei observed that "North Korea wanted to engage directly with the United States and was therefore acting like a 'spoiled child' in order to get the attention of the 'adult'. China encouraged the United States, 'after some time', to start to re-engage the DPRK," according to the diplomatic cable sent to Washington. 
 
 A second dispatch from September last year described He downplaying the Chinese premier's trip to Pyongyang, telling the US deputy secretary of state, James Steinberg: "We may not like them ... [but] they [the DPRK] are a neighbour." 
 
 He said the premier, Wen Jiabao, would push for denuclearisation and a return to the six-party talks. The official also complained that North Korea "often tried to play China off [against] the United States, refusing to convey information about US-DPRK bilateral conversations". 
 
 Further evidence of China's increasing dismay with Pyongyang comes in a cable in June 2009 from the US ambassador to Kazakhstan, Richard Hoagland. He reported that his Chinese counterpart, Cheng Guoping. was "genuinely concerned by North Korea's recent nuclear missile tests. 'We need to solve this problem. It is very troublesome,' he said, calling Korea's nuclear activity a 'threat to the whole world's security'." 
 
 Cheng said Beijing "hopes for peaceful reunification in the long term, but he expects the two countries to remain separate in the short term", Hoagland reported. China's objectives were "to ensure they [North Korean leaders] honour their commitments on non-proliferation, maintain stability, and 'don't drive [Kim Jong-il] mad'." 
 
 While some Chinese officials are reported to have dismissed suggestions that North Korea would implode after Kim's death, another cable offers evidence that Beijing has considered the risk of instability. 
 
 It quoted a representative from an international agency saying Chinese officials believed they could absorb 300,000 North Koreans without outside help. If they arrived "all at once" it might use the military to seal the border, create a holding area and meet humanitarian needs. It might also ask other countries for help. 
 
 The context of the discussions was not made explicit, although an influx of that scale would only be likely in the event of regime failure. The representative said he was not aware of any contingency planning to deal with large numbers of refugees. 
 
 A Seoul embassy cable from January 2009 said China's leader, Hu Jintao, deliberately ducked the issue when the South Korean president, Lee Myung-bak, raised it at a summit. 
 
 "We understand Lee asked Hu what China thought about the North Korean domestic political situation and whether Beijing had any contingency plans. This time, Hu apparently pretended not to hear Lee," it said. The cable does not indicate the source of the reports, although elsewhere it talks about contacts at the presidential "blue house" in South Korea.
– Don't expect pressure from China to have an effect on North Korea's nuclear weapons program. That was the message in an editorial published by the North's state news media Wednesday, described by an expert as the harshest critique of China to come out of the country in recent memory, per the New York Times. The commentary, with the byline "Kim Chol," said the weapons program is as precious to North Korea "as its own life" and the "line of access to nukes for the existence and development of the country can neither be changed nor shaken," no matter what China does. It adds that the program will continue even if it means the end of North Korea-China relations, though it warns China that such a scenario would bring "grave consequences." Other commentaries critical of China have appeared in North Korea's state news media recently, but none were so pointed or identified China by name. It isn't clear if North Korea would actually give up its ally, which supplies almost all of its external trade and oil imports, but one expert suggests the country could make do with stronger ties to Russia and South Korea. Now might actually be a good time: The leading candidate for president in South Korea, Moon Jae-in—who met Kim Jong Un's father a decade ago—appears willing to soften tensions, reports the Wall Street Journal. A rep for China's Foreign Ministry, however, says China remains committed to "developing friendly, good-neighborly relations with North Korea," per Reuters.