China, long viewed as North Korea's protector, increasingly doubts its own influence and would support the peninsula's reunification if the regime collapses, leaked US documents said Monday.

Over an expansive dinner last year, the Chinese ambassador to Kazakhstan revealed that Beijing considers North Korea's nuclear program to be "very troublesome," according to a memo obtained by whistle-blower site WikiLeaks.

Ambassador Cheng Guoping "said 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 Britain's The Guardian newspaper.

In another cable reproduced by The New York Times, a Chinese official whose name was removed said that Beijing believed North Korea had "gone too far" after carrying out its second nuclear test and firing a missile.

The official told a US diplomat "that Chinese officials had expressed Chinese displeasure to North Korean counterparts and had pressed (North Korea) to return to the negotiation table," it said.

"Unfortunately," the Chinese official was quoted as saying, "those protests had had no effect."

"'The only country that can make progress with the North Koreans is the United States,'" the Chinese official was quoted as saying.

WikiLeaks has outraged the US government with its massive release of sensitive data. The memos became public a week after North Korea shelled a South Korean border island, killing four people and sending tensions soaring.

The standoff comes as North Korea's reclusive leader Kim Jong-Il, who has suffered a stroke, prepares to hand over power to his little-known youngest son Kim Jong-Un, who is believed to be in his late 20s.

Dai Bingguo, China's state councilor, is quoted in a cable as telling US officials after a visit to Pyongyang that Kim Jong-Il had lost weight but "appeared to be in reasonably good health and still had a 'sharp mind.'"

Kim said he still drank alcohol, saying that only scheduling problems stopped the North Korean leader from partaking with his guest in one of his legendary drinking sessions, according to the cable.

Many US experts believe that China wants to preserve the status quo on North Korea, fearing that a collapse would trigger a flood of refugees and bring a united and US-allied Korea to its border.

But senior South Korean official Chun Yung-Woo is quoted in a cable as saying that more "sophisticated" Chinese officials have come to believe that North Korea "has little value to China as a buffer state" since its first nuclear test in 2006.

Chun also said that South Korea believed that North Korea "had already collapsed economically" and would "collapse politically" two to three years after Kim's death.

Chun, who was then vice foreign minister and is now national security adviser, said 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 (North Korean) leadership knows it," he said.

However, another cable quoted a Chinese official who "discounted strongly any suggestion that the system would collapse once Kim Jong-Il disappeared."

The unnamed official "cautioned that US experts should not assume North Korea would implode after Kim Jong-Il's death," it said.