China will eventually abandon traditional ally North Korea, according to United States documents released by whistleblowing website WikiLeaks.

China is fed up with North Korea and now sees the regime as of less value as a buffer state. That is the assessment contained in the leaked diplomatic cables.

The documents include comments attributed to China's ambassador to Kazakhstan.

Over an expansive dinner last year, Cheng Guoping apparently said Beijing considers North Korea's nuclear program to be "very troublesome".

He also reportedly said China would accept unification of the Korean peninsula.

"China hopes for peaceful reunification in the long-term, but he expects the two countries to remain separate in the short-term," said the leaked cable by US ambassador Richard Hoagland and reprinted by The Guardian newspaper.

In another document, South Korea's former vice foreign minister, Chun Yung-woo, said the younger generation of Chinese leaders "no longer regarded North Korea as a useful or reliable ally".

Mr Chun added that China "had far less influence on North Korea than most people believe".

"Beijing had 'no will' to use its economic leverage to force a change in Pyongyang's policies and the DPRK leadership knows it," he said, referring to the North's official name, the Democratic People's Republic of Korea.

Mr Chun also said South Korea believed North Korea "had already collapsed economically" and would "collapse politically" two to three years after the death of leader Kim Jong-il.

Mr Kim, who is now embroiled in tensions with the South, is believed to be preparing to hand over power to his son, Kim Jong-un, who is in his 20s.

The leaked comments point to a major shift in Beijing's position.

China has traditionally been seen as the most important, and at times only backer of North Korea, with some suggesting that if China was to pull the pin, North Korea would collapse.

Analysts have been wondering for years how long the Chinese leadership would provide a buffer for North Korea.

Opinions in state-controlled media have also shifted this week, with some blaming North Korea for the current crisis on the Korean peninsula.

However, there are concerns the release of the documents could spark a backlash, whereby people who are in favour of stronger connection with North Korea try to undermine those advocating within the Chinese government to cut ties with the North.

China has called for emergency six-party talks in the wake of the North's attack on the peninsula.

- ABC/wires