As North Korea stokes nuclear tensions to crisis point, China is being frustrated by its defiant yet dependent "younger brother" running roughshod over its desire for regional stability, analysts say.

Beijing could wield tremendous leverage over the isolated regime, courtesy of the vital aid it provides, including almost all of North Korea's energy imports, and protection from tougher global sanctions.

But while Beijing is Pyongyang's sole major ally it is constrained by fear that antagonising the regime could unsettle the region, leaving it to watch as Pyongyang flouts its repeated calls for calm and restraint.

Under its young leader Kim Jong-Un, Pyongyang has ratcheted up the crisis almost daily since a February nuclear test, reacting angrily to fresh UN sanctions by threatening an atomic strike on the South and United States.

China "may expect the behaviour of North Korea to be like a younger brother, but the new leader Kim Jong-Un didn't follow such expectations", said Kim Heungkyu, a professor at Sungshin Women's University in Seoul. "It really bothers the Chinese."

Kim Jong-Un ascended to power in the world's only Communist family dynasty following the death of his father in 2011, but has yet to visit its giant neighbour as a way to "pay a kind of special respect to China", said Kim.

He attributed the younger Kim's audacity to inexperience, saying his father and predecessor handled China in a more "traditional" way.

Jia Qingguo, an international relations expert at Peking University, added: "What he has been doing is to push all sides, and he has showed defiance to China as well as to the US, Japan and South Korea."

But China fears instability could bring refugees flooding across the border, an enhanced US military presence in the region or ultimately even a united pro-US Korean peninsula.

"More and more people in China are beginning to question the traditional assumption that North Korea can be a strategic buffer zone for China," said Jia, but added: "Don't expect too much. China wants to have a good relationship with North Korea."

Beijing has backed Pyongyang since the 1950-53 Korean war. It is estimated to provide as much as 90 percent of its neighbour's energy imports, 80 percent of its consumer goods and 45 percent of its food, according to the US-based Council on Foreign Relations.

While China voted in favour of recent UN sanctions, it negotiated to soften them first, and its trade and aid appear to be continuing.

"The North Koreans know that they've put themselves in this position where no one can reach them unless they're willing to use force or see their economic collapse," British historian Lawrence Freedman said in Hong Kong on Wednesday.

Defenders of China's policy argue that North Korea plans to develop its nuclear weaponry anyway -- in a rational calculation to protect itself through deterrence -- and punitive measures could simply provoke a dangerous reaction.

They also counter that the true powerbroker is the US, as it can offer the regime its prized goal of a security guarantee.

Without naming any countries, President Xi Jinping said at an international gathering at the weekend that "no one should be allowed to throw a region and even the whole world into chaos for selfish gains".

A day earlier China's Foreign Minister Wang Yi said it "will not allow troublemaking on China's doorstep".

The official remarks might be aimed at creating a little distance from Pyongyang -- but how that could translate into action remained unclear, said Roderic Wye, a fellow at Chatham House in Britain.

"It is a clear sign that the Chinese are much more concerned," he said. "It's not a break with North Korea, but it's making it clear to the North Koreans that they need to consider the consequences of what they are doing.

"The question is, what is China going to do about it? How are they going to demonstrate their concern other than rhetorically?" he said.

"They are in a dilemma, basically. It is going badly."

Wang Dong of Peking University said that Xi's comments may have also been meant to signal displeasure toward the US.

"The Chinese side has been very frustrated by North Korea," he said, but added that "you can interpret the speech in both ways".

The ambiguous wording would be consistent with Beijing's longstanding calls for all sides to avoid provocation, Wang said, adding that "the speech writers must be working very hard. This is really a piece of artwork".