While the world watches North Korea test yet another missile in defiance of international sanctions and warnings, China continues to be a friend to Kim Jong Un’s regime.

The relationship has prompted questions over how far Pyongyang would have to go before Beijing turned on its neighbor.

After this week’s latest missile test, again over the Sea of Japan and which North Korea claimed could have reached the United States mainland, the U.S. ambassador to the United Nations warned the regime that it could be “utterly destroyed” if its provocations started a military conflict.

Meanwhile the world’s other superpower China criticized its neighbor but in far more measured tones, saying that while it had “grave concern and opposition” to the launch, it wanted to “preserve peace and stability” in the region.

As North Korea’s nearest neighbor, China has a vested interest in the North Korea’s stability but the relationship is crucial for North Korea too with China being the impoverished nation’s biggest trading partner, giving a lifeline to the isolated economy.

North Korea is subject to many United Nations (UN), U.S. and European sanctions due to its continued development of nuclear weapons and ballistic missiles. The state is seen as a serious threat to international peace and security and its actions as undermining a global agreement on the non-proliferation of nuclear weapons.

Paul Haenle, director of the Carnegie–Tsinghua Center based at Tsinghua University in Beijing, told CNBC on Thursday that he didn’t think the latest missile test “would change China’s position on North Korea.”

“While China continues to want a denuclearized peninsula, stability is its first priority. China prefers to live with a nuclear-powered but friendly neighbor to one with only conventional weapons, but that is unfriendly,” he said.

“What would change that calculus is if North Korean nuclear tests began to impact the safety or environment of China’s northeast, thereby threatening Chinese domestic and social stability. With President Trump’s visit behind them, I don’t see Chinese leaders feeling the same urgency to step up pressure on North Korea outside of the normal United Nations process,” he said.

In further remarks on Wednesday, U.S. Ambassador to the UN Nikki Haley said that the U.S. could disrupt Chinese crude oil shipments to North Korea, although Beijing has so far refused to cut off energy exports to Pyongyang.

While China has consistently supported UN sanctions against North Korea, one expert said it had an ambivalent relationship with its southern neighbor and that Sino-North Korean relations had deteriorated since Kim Jong Un came to power in 2011.

“China’s priority is stability in the region, as well as domestic stability, and part of that is an alliance with its neighbor North Korea,” Alison Evans, deputy head of Asia-Pacific country risk at IHS Markit, told CNBC on Thursday.

“However, North Korea is a bit like a belligerent little brother causing trouble for China,” she said, noting that a number of North Korea’s missile tests this year had occurred during China’s hosting of high-profile events.

“By doing this, North Korea has caused China to lose face and it just shows that North Korea is not behaving in a way that keeps its allies in mind,” she said.

Implementing sanctions, then, has been a way for China to “punish” North Korea and yet it has suggested it was prepared to defend the country if the U.S. attacked it, according to an editorial in Chinese state run newspaper The Global Times.

Evans noted that, ultimately, for China it was important to secure its strategic interests, and that meant stopping U.S. influence spreading in the region.

“China would defend North Korea because it’s an ally and because it’s in China’s strategic interests to have an allied Communist state as a buffer state rather than a Korean peninsula unified under a South Korean flag,” she said.

China is not the only country ready to defend North Korea from foreign, and particularly U.S., interference. Foreign Minister Sergey Lavrov said Thursday that the U.S. could have deliberately provoked North Korea to take tough actions, according to Russian state news agency TASS.

“Washington’s recent steps seem to be deliberately aimed at provoking Pyongyang to take some tough actions,” he said. The U.S. should openly say if its provocative actions are aimed at destroying North Korea,” he added, according to the news agency.