Chinese President Xi Jinping, left, and North Korean leader Kim Jong-un wave from an open top limousine as they travel along a street in Pyongyang during Xi's visit in this June 20, 2019, photo. AP-Yonhap

Only a few days remain until Oct. 6, which marks the 75th anniversary of the establishment of diplomatic relations between China and North Korea.

However, both countries have remained silent, not mentioning the upcoming anniversary or revealing any plans to commemorate it.

Experts view this as another indication of strained Sino-North Korea ties.

"Their bilateral relations over the past decades were marked by ups and downs. Most of the time, they were not on good terms," Choo Jae-woo, a professor of China Studies at Kyung Hee University, said. "Interestingly, however, their strained ties have managed to be restored each time. Given this dynamic, it would be a mistake to try to interpret current events as a means to predict the future of their relations."

As Mao Zedong likened Sino-North Korea relations to "lips and teeth," the two countries remain closely connected and dependent on each other. North Korea relies on China, its largest benefactor, for survival, while China continues to support its unpredictable and impoverished neighbor to prevent its collapse. If North Korea were to collapse, China would have to confront the reality of a free, unified Korea with U.S. troops stationed along its border.

Despite their interdependence, China and North Korea lack trust in each other.

A couple of months before then-North Korean leader Kim Jong-il died of heart disease on December 17, 2011, he prepared a long list of dos and don'ts for his successor and youngest son, Jong-un. The list included nearly 40 items related to domestic politics, security, foreign policy, and more.

Regarding China, the late Kim's message was clear: while North Korea needs China, he cautioned that China is not dependable, advising his son not to place trust in it.

He blamed China for causing North Korea some of its most difficult times.

A North Korean defector named Lee Yoon-geol shared details of the late Kim's will, claiming he obtained the information from multiple sources within North Korea who had access to it.

The late Kim is not the only North Korean leader to express deep-seated suspicions of China. His father, Kim Il-sung, the founder of North Korea, was also critical of China. This distrust has been passed down to the current leader, Kim Jong-un.

In his memoir, titled "Never Give an Inch: Fighting for the America I Love," former U.S. Secretary of State Mike Pompeo wrote that Kim referred to the Chinese as "liars" and expressed his need for U.S. forces to protect him from China.

In a speech to the Asia Society in New York in October 2017, Ri Jong-ho, a former North Korean economic official who worked at "Office 39" — a secretive state apparatus managing foreign currency slush funds to finance the Kim family's luxurious lifestyle and North Korea's weapons of mass destruction program — revealed that Kim used a swear word to refer to Chinese leader Xi Jinping during a meeting with high-ranking officials.

Kim's harsh criticism of the Chinese leader came shortly after Xi visited South Korea in 2014 for a summit with then-President Park Geun-hye.

Sino-North Korea relations have soured throughout this year.

North Korea expressed dissatisfaction with China's plan to establish 191 telecom facilities capable of broadcasting FM signals, including 17 stations near the border. In an email to the International Telecommunication Union (ITU), North Korea claimed that China did not consult with it about the plan beforehand. The ITU dismissed the North's complaint, stating that registering FM stations with the ITU or bringing them into service does not require prior agreement among countries.

North Korea reacted furiously to the plan because, if those stations were to be established as scheduled, the influx of outside information into the country would be inevitable.

"The reason that Pyongyang might object to the stations is that FM broadcasts could undermine the regime," Joel Atkinson, a professor at Hankuk University of Foreign Studies, said. "Pyongyang knows that a complaint to the ITU will not change Beijing's mind. So, the only purpose is to alert the outside world to a point of friction. I suspect it is part of a disinformation campaign, a campaign which is clearly working."

Atkinson suggested that the dispute could be resolved, as both sides share a common interest.

If the FM signals are indeed a threat to the regime, he noted that China and North Korea would have a shared interest in preventing that, as both seek to avoid a regime change in the North.

Earlier this year, media reports indicated that a footprint tribute commemorating the friendship between the Chinese and North Korean leaders, established during Kim's visit to Dalian, China, in May 2018, was removed. Months later, tensions between the two countries escalated when China informed all North Korean guest workers in the country to return home. North Korea proposed a gradual return of those workers while allowing new workers to replace them, but China rejected this offer.

About 100,000 North Koreans are known to work in China, and the remittances they send back home are used to finance North Korea's missile and nuclear programs.

In this pool photograph distributed by the Russian state agency Sputnik, North Korean leader Kim Jong-un, second from right, shakes hands with Russian President Vladimir Putin, second from left, during a welcoming ceremony at Kim Il Sung Square in Pyongyang on June 19. AFP-Yonhap

Russia effect

North Korea's relations with China have soured amid the North's increasing military ties with Russia following the war in Ukraine. This rare coincidence has fueled speculation about the possible "Russia factor" influencing the strained relations between Pyongyang and Beijing.

Kang Cheol-hwan, a North Korean defector, claimed that the cooperation between Pyongyang and Moscow could have negatively affected the North's relations with China.

He noted that Chinese officials probably felt slighted when observing North Korean leader Kim Jong-un's differing treatments of Chinese and Russian officials during an event held last year in Pyongyang to commemorate the 70th anniversary of the end of World War II.

Kim warmly welcomed the Russian delegation led by then-Defense Minister Sergei Shoigu and treated him very nicely. In contrast, Kim did not even make eye contact with the Chinese officials, emphasizing that the Chinese would have certainly recognized the North Korean leader's differing reactions to officials from the two countries.

Experts who share Kang's perspective argue that China is worried about losing its influence over North Korea to Russia. They suggest that increased support from Russia could embolden the North to make miscalculations, potentially leading to actions that might destabilize the Korean Peninsula.

However, other experts contend that the negative impact of military cooperation between Russia and North Korea on Sino-North Korea ties is unlikely, emphasizing that the three autocratic nations share strategic interests.

"China views the new Russia-North Korea partnership as convenient and perhaps as a new model for how to confront the West with proxies," said Chris Monday, a professor at Dongseo University in Busan. "With the end of Pax Americana, the world situation is unprecedented. China has not yet developed a fixed strategy. China wants Putin's Russia as a partner against the West but is reluctant to tie its fate to such an unpredictable ally."

Atkinson said the three nations are aligned in their cooperation, suggesting that military deals between Russia and Pyongyang will not affect China's approach toward North Korea.

"My view is that Beijing sees China, Russia and North Korea all working toward a common goal: the reduction of U.S. power and influence," he said. "Therefore it tacitly supports Russia-North Korea cooperation but does not support it publicly as that would be counterproductive in terms of generating a larger reaction from the U.S., South Korea, Europe and others. This would not only be bad for China, but for Russia and North Korea as well. In other words, messages that China is against it is part of a disinformation campaign that helps the three countries achieve their common goal."

Regarding the Russia factor, Choo mentioned that China won't feel any pressure from the growing North Korea-Russia relations, noting that there is a consensus between China and Russia concerning North Korea.

"China's top foreign policy priority is placed on improving its relations with the U.S.," he said. He added that China knows how it should react to North Korea because the country would not gain anything if it is seen as being close to or trying to assist the provocative North Korea," he said. "China would feel relieved as Russia economically helps North Korea."

North Korean leader Kim Jong-il, left, is seen touring Shanghai with Chinese Premier Zhu Rongji in this image from Chinese state television, dated January 20, 2001. AP-Yonhap