The decades-long strategic relationship between Beijing and Pyongyang is looking increasingly strained.

A sure sign of that decline can be seen in a Wednesday message from North Korean leader Kim Jong Un to Chinese President Xi Jinping, according to Kim Tae-hyo, political science professor at Seoul-based Sungkyunkwan University.

The dictator wished Xi “great success,” according to state media, as the latter received a second five-year term at the 19th Party Congress. But the overall tone was less cordial than previous correspondence.

When Xi was first appointed five years ago, Pyongyang sent a six-sentence-long congratulatory message that included phrases such as “strength of the mutual leadership, friendship and brotherhood,” said Sungkyunkwan University’s Kim. This time, the note was only four sentences and made no mention of brotherhood or friendship, he continued.

Traditionally reluctant to coerce Pyongyang, but under heavy pressure from Washington, the world’s second-largest economy has been increasingly clamping down on its historical ally. Beijing has banned domestic lenders from doing business with North Korean clients in addition to halting certain exports in compliance with international sanctions — recent figures, however, showed a 3.7 percent annual increase in bilateral trade during the first nine months of the year.

The Chinese are increasingly realizing the North’s nuclear program is harming Asian security, Renmin University associate professor Cheng Xiaohe told the Carnegie-Tsinghua Center for Global Policy in a recent podcast.

But tensions between Xi and Kim aren’t new.

Since Kim came to power, the bilateral relationship has entered a period of abnormal relations marked by a decline in economic ties and lack of political solidarity, Cheng said.

In fact, Beijing was actually disappointed when the 33 year-old was installed as leader of the secretive state in 2011, Sungkyunkwan University’s Kim said.

That is, Beijing did not want to see North Korea governed by a third-generation grandson, he explained. Chinese policymakers believe collective leadership would work best in the rogue state, instead of the supreme power currently enjoyed by Kim Jong Un, he continued.

The isolated ruler has purged many supporters of his father Kim Jong Il and he further consolidated political power earlier this month by promoting his younger sister to a top post.

Kim’s style of governance also differs from his father, who was interested in reaping economic concessions through international negotiations and never pointed North Korean missiles toward Washington, the South Korean professor noted. The current Kim, however, has showed no desire for diplomatic talks.