Despite being North Korea's biggest ally, China has seen its alliance with the regime repeatedly tested in recent years – and the past week is no exception. With a ballistic missile test this past Sunday, and the apparent assassination of Kim Jong-nam, the exiled half-sibling of Supreme Leader Kim Jong-un, the relationship between the two nations is growing increasingly tense.

On Monday, Kim Jong-nam died en route to the hospital from Kuala Lumpur International Airport after he claimed he was sprayed with a chemical. South Korean and US officials have pointed to North Korea as the culprit behind the apparent assassination, and three suspects – one Malaysian, one Indonesian, and one Vietnamese – have been taken into custody.

After North Korea's latest long-range missile test on Feb. 12, China showed its displeasure by rejecting a shipment of its coal worth $1 million. On Saturday, Beijing announced its decision to ban all coal imports from North Korea until the end of 2017, according to BBC. Coal is Pyongyang's biggest export and a crucial source of revenue for the government.

“China's relationship with North Korea in recent years has become increasingly estranged,” Bonnie Glaser, a senior adviser for Asia at the Center for Strategic & International Studies, tells The Christian Science Monitor. “Contacts continue, fairly stable ... but I think it's clear that China is not very pleased with North Korea's policies," particularly its nuclear weapons program. But on the other hand, "the North Koreans are also dissatisfied with some of China's behavior, often portraying China as cooperating with the United States and other countries against it. There's a lot of friction in the relationship.”

For decades, China's political and economic support has helped sustain its neighbor's regime. Beijing has feared that if North Korea collapsed it could create a refugee crisis, and it wants to keep North Korea as a buffer between itself and South Korea, a close US ally. The relationship has never been friction-free, Ms. Glaser emphasizes, but it has met greater challenges since the young new leader, Kim Jong-un, took office in 2011.

Ever since the "Hermit Kingdom" first announced a successful nuclear test, in 2006, “there has been a sense in China that North Korea is defying Beijing and Chinese interests,” she says. “I think that the North Koreans' unwillingness, particularly under Kim Jong-un, to even consider going back to the pledge of denuclearization, that has really exacerbated the tension in the relationship.”

Now, recent events in the region could threaten to highlight that discord even more. Kim Jong-nam, who had been living in the Chinese territory of Macau, was generally assumed to be under the protection of the Chinese government. His sudden death on in a Malaysian airport on Tuesday, which many speculate was decreed by Pyongyang, is “a blow to Beijing,” John Power reported for The Christian Science Monitor on Wednesday.

For the Chinese government, the elder Kim was valuable as a potential future leader, if his half-brother's regime ever collapsed, Robert Kelly, a politics professor at Pusan National University in South Korea, told Mr. Power. That made Kim Jong-nam, however "a constant regime leadership replacement threat" to the current Supreme Leader.

The death has led to “a sense of shock and dismay” in China’s inner circle of government, according to The Washington Post, as officials interpret the possible assassination orders from Pyongyang as an affront to the two countries' alliance.

But North Korea’s actions are careful calculations, says Andrew Scobell, a senior political analyst at the RAND Corporation, a global policy research institute.

Kim Jong-un “wants to demonstrate that he is an independent actor, he is not anyone's puppet, his regime is not a client state of any country,” Dr. Scobell tells the Monitor. “Clearly, China is the country that has closest ties to North Korea in terms of economics, or anything for that matter, so it's the one country that North Korea's leader wants to emphasize that they have independent autonomy from.”

Scobell adds that Kim Jong-un’s calculated but risky choices, ranging from frequent nuclear missile tests to the executions of more than 340 people – including his uncle, once among the country's top officials – have reminded China of the regime's volatility and unpredictability, and put China in a “catch-22” situation.

“Beijing is fearful of reacting too harshly, for example, cutting off all trade with North Korea, which they could do,” he says. “But in Beijing, there's concern that if they crack down too hard on North Korea, then the situation will get even worse.”

While China supported UN sanctions against North Korea last November, and has previously shut off its oil pipeline to the country, these efforts are not sustained or tough enough to prompt real change, in Scobell's option.

Beijing has consistently provided food and financial assistance to Pyongyang, but North Korea actually has the upper hand in the relationship, Glaser notes, as they use China’s interest in preserving stability in the region as leverage.

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Yet, going forward, Scobell suggests the recent murder of Kim Jong-nam could be a turning point for the relationship between the two countries, as the North Korean leader might feel more secure about his power.

"It's hard to say," he says, but if Kim Jong-nam's apparent assassination "reassures" his half-brother, "this may pave the way for a change in policy perhaps towards China."