As Kerry heads to Seoul, South Korea, tensions with North Korea continue to rise as it remains unclear whether or not the latest rhetoric is merely Kim Jong-un showing off his military strength. NBC's Andrea Mitchell reports.

News Analysis

BEIJING -- There was confusion at the China-North Korea border Thursday after Chinese tour operators halted trips into the North.

Wang Zhao / AFP - Getty Images Two men wait Thursday for dispatch at a customs port in the Chinese border city of Dandong. The largest border crossing between North Korea and China has been closed to tourist groups, a Chinese official said Wednesday.

It wasn't clear whether the instruction to do so came from the Chinese authorities, the North Koreans, or was made by the nervous operators themselves.

But it mirrored a wider confusion over Chinese policy toward Pyongyang, which depends on Beijing for food and fuel, as well as diplomatic support.

As North Korea readies what is thought to be a missile test, China's Foreign Ministry spokesman Hong Lei has spent most of the week deflecting questions with the official line that "all sides" should show restraint and begin dialogue, and that peace and stability are a "shared responsibility."

But in an interview with NBC News he was more forthright about China's growing concern. "We do not want to see chaos and conflict on China's doorstep," he said.

In fact, there are signs that China is rethinking its policy toward the North. President Xi Jinping last weekend told a forum of political and business leaders that no country "should be allowed to throw a region and even the whole world into chaos for selfish gain." He didn't mention the North by name, but it was pretty clear who he was referring to.

Defense Secretary Chuck Hagel described North Korea's actions and "bellicose rhetoric" as "skating very close to a dangerous line." &nbsp;NBC's Richard Engel reports.

Earlier, China's Foreign Minister Wang Yi had told UN Secretary-General Ban Ki-moon that Beijing would not allow "troublemaking on China's doorstep," a line repeated in an editorial in Thursday's China Daily.

China also supported the latest UN sanctions that followed North Korea's third nuclear test.

In fact, relations between the two have been souring for some time as Pyongyang has consistently ignored calls by Beijing for restraint.

"To many in Beijing, North Korea is looking less like a strategic asset and more like a strategic burden," said Cheng Xiaohe, associate professor at Renmin University's School of International Studies.

In the past, even when clearly unhappy, Beijing has treated the North with kid gloves because of fear of the North collapsing, and also as a hedge against U.S. power in Asia.

'Little Fatty'

According to leaked 2010 diplomat cables obtained by Wikileaks and posted by newspapers the Guardian and the New York Times, Chinese officials described the regime in the North as behaving like a "spoiled child."

Slideshow: North Korea's young leader, Kim Jong Un The youngest son of Kim Jong Il succeeded his late father in 2011, becoming the third member of his family to rule the unpredictable and reclusive communist state. Launch slideshow

Chinese social media, which is as close a barometer of public opinion as you can get here, has in recent days been buzzing with criticism -- not of the U.S., but of North Korean leader Kim Jong Un, for leading his country to disaster and the world close to war.

Kim is derided as "Little Fatty" or "Fatty the Third."

One former top U.S. diplomat agrees there are clear signs that China is losing patience with North Korea. Kurt Campbell, the state department's top official for east asia, said there are signs that a relationship once described by Chairman Mao to be "as close as lips and teeth" is wearing thin.

He said this was notable in public statements and private conversations with U.S. officials. Speaking last week at a forum at Johns Hopkins University, he said this had the potential for a large impact on northeast Asia.

What's harder to say is how this growing frustration will be translated into concrete actions to pressure the North.

Cheng of Renmin University noted that in 2003 Beijing turned off the oil supply in order to force Pyongyang to join six-party talks and could use that weapon again.

Secret filming captures N. Korean smugglers sneaking into China to get supplies for their impoverished country, as a refugee tells of the horror of life under Kim Jong Un. ITN's Angus Walker reports.

"If China has political will, China can do something," he said. "China can make a difference."

Secretary of State John Kerry will be taking this up with China's leaders when he is there this weekend.

"China and the U.S. share common interests in peace, stability and denuclearisation," said the Foreign Ministry's Hong Lei. "We hope to work with the U.S. side towards that end."

Significantly, there has so far been no Chinese criticism of the display of U.S. high-tech firepower in the region, which is seen as another tacit condemnation of Pyongyang's antics.

That said, Kerry will no doubt point out, as other officials have done privately, that if China fails to act the result will be an even bigger U.S. military presence in the region and a possible regional arms race -- precisely what China has said it wants to avoid.

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Full North Korea coverage from NBC News