BEIJING — For decades, North Korea could count on China as a loyal ally despite the erratic behavior of the ruling Kim dynasty. Beijing held its tongue, even as its neighbor engaged in saber rattling, nuclear testing and bombastic threats.
But by this week, things may have gone too far, with North Korea unleashing a tirade in which it deployed some of the most damning insults in its playbook, accusing China of “dancing to the tune of the U.S.” and “styling itself as a big power.”
Chinese observers of relations between the countries were left in disbelief about how rapidly those ties have deteriorated, particularly after Beijing decided to cut off coal imports that provide badly needed currency for the North’s economy.
“At first many people thought it was a fake commentary,” Cheng Xiaohe, associate professor of international studies at Renmin University, said of the tirade against China, published in the North’s state-run news media on Thursday. “It is a big surprise that North Korea has launched such a strong attack against China. I expected an angry reaction — but not this strong.”
The diatribe, carried by the Korean Central News Agency, came just before the Malaysian government announced that VX nerve agent had killed Kim Jong-nam, the half brother of the North’s leader, Kim Jong-un. North Korean agents are suspected of masterminding the attack.
The disclosure that a chemical weapon banned under international treaties was used in the attack is sure to put more pressure on China’s relations with North Korea, which some Chinese analysts say are at their lowest point since the founding of the North as a separate country after World War II.
The disclosure “makes things worse,” Mr. Cheng said. “It’s wrong to kill the half brother. It’s more wrong to have used banned chemical agents.”
Earlier this month, the North issued a more indirect takedown of Beijing in its government newspaper Minju Joson, signaling a growing rift. Shortly afterward, China announced the suspension of coal imports.
The burst of criticisms from Pyongyang — coupled with Beijing’s coal ban — suggested boiling tensions between China’s president, Xi Jinping, 63, who sees himself as a global leader, and Mr. Kim, 33, an eccentric dictator.
Mr. Xi is said to have low regard for Mr. Kim, who has not visited China and is not known to have been invited.
Despite past periods of turbulence, including under Mao Zedong, both sides have more or less tried to preserve a polite public veneer of amity. But the friendship was a myth, said Shen Zhihua, a professor of history at East China Normal University.
The Chinese government has always viewed North Korea as a “vassal country,” he said in an interview with Phoenix Television on Thursday, shortly before the North Korean editorial was published.
“You can go through the People’s Daily from 1949 to this day, we have never said a single word of ill, all words of praise,” about North Korea, said Mr. Shen, who is one of China’s pre-eminent historians on the North. “Meanwhile, the North Korean newspapers do not speak ill of China.”
Even as the relationship has noticeably worsened in recent weeks, China has maintained public restraint. An editorial in Global Times, a state-run tabloid that sometimes reflects the thinking of senior government officials, noted on Friday that the suspension of coal imports was “fair” and in keeping with United Nations sanctions.
The Chinese Foreign Ministry also kept up that facade. “China and North Korea are friendly neighbors, and we are willing to work with the North Korean side in joint efforts to develop healthy and stable relations,” said Geng Shuang, the ministry spokesman.
The ministry also played down the significance of the Malaysian announcement that VX had been used in the killing of Kim Jong-nam.
“It is only a preliminary result published by Malaysia, and there is not conclusion on this yet,” Mr. Geng said.
It was not immediately clear how much China’s ban on coal imports would affect North Korea’s ability to look after its population. And at least publicly, China said that it was imposing the ban only because it had already fulfilled its coal quota allowed under United Nations sanctions.
That seemed to suggest that Beijing may have already paid Pyongyang for the coal it imported in the first 50 days of this year, money that would go to the North’s cash-starved government, experts said.
The exposure of the deepening rift comes as the Trump administration has been pressuring Beijing to use its leverage to curtail the North’s rapidly expanding nuclear weapons program.
In the last week, the American secretary of state, Rex Tillerson, has spoken with China’s senior foreign policy official, Yang Jiechi, and the Chinese foreign minister, Wang Yi, about North Korea.
President Trump has said China can do significantly more to persuade the North to scale back its nuclear program, even as Beijing has told Washington that it holds limited influence. The latest flare-up is likely to further weaken that leverage, while illustrating the resistance of Kim Jong-un to China’s arguments in favor of a denuclearized Korean Peninsula.
“In terms of diplomacy, I see no solution,” said Yan Xuetong, the director of the Institute of International Studies at Tsinghua University. Mr. Yan, writing in the Chinese news media this week, said that China had no choice but to accept the North as a nuclear-armed state. That was because China had only two options: either a nuclear North Korea that was friendly toward China or a nuclear North Korea that was unfriendly, he wrote.
“I am very pessimistic about this issue,” Mr. Yan said.