China has expressed frustration with North Korea, with one official calling it a "spoiled child," at the same time as brushing off U.S. requests to choke off the flow of military technology from Pyongyang to Tehran that helps to sustain the regime, according to leaked U.S. diplomatic cables.

One of the latest batches of cables released by WikiLeaks also quotes a South Korean official speculating that China might be able to accept reunification under South Korea's leadership.

Another cable quotes a Chinese official telling a U.S. Embassy official after North Korea's missile test last year that: "North Korea wanted to engage directly with the United States and was therefore 'acting like a spoiled child' to get the attention of the "adult." The Guardian, one of five news organization that got early access to the cables, names the Chinese official as Vice Foreign Minister He Yafei.

Another cable, according to the Guardian, described Mr. He in September 2009 playing down a visit by Chinese Premier Wen Jiabao to Pyongyang, telling the U.S. deputy secretary of state, James Steinberg: "We may not like them.…[but] they [North Korea] are a neighbour."

One cable cites a Chinese ambassador saying the North's nuclear activity is "a threat to the whole world's security."

The cables offer a unique window on the extent of China's frustration with its old Communist ally, which triggered a fresh crisis on the Korean peninsula when it shelled a South Korean island last week.

But they also reflect continuing U.S. concern that China isn't doing enough to prevent proliferation of materials and technology, including some to and from North Korea, which could help Iranian weapons programs.

China declined to act on multiple U.S. requests that it stop shipments of ballistic-missile components from North Korea to Iran on commercial flights via the Beijing airport in 2007, according to one of the first batch of cables made public Sunday.

Another cable showed that U.S. Secretary of State Hillary Clinton asked China in February to act on intelligence that Iran was trying to buy gyroscopes and carbon fiber for its ballistic missiles from Chinese companies. According to one more cable, Mrs. Clinton expressed concern in May that Chinese companies were supplying Iran with precursors for chemical weapons.

While diplomats and members of the intelligence community have long spoken of China's alleged role in helping to develop Iran's nuclear and missile programs, the newly released cables put that relationship in its clearest relief yet.

"The cables are revealing because it shows where Iran gets both its parts and its technology," said David Albright, head of the Institute for Science and International Security, a Washington think tank. "Iran is using China for both."

China pledged in 2000 not to help any country develop ballistic missiles that can be used to deliver nuclear weapons. China also introduced stricter export controls in 2002 and in June applied to join the 34-country Missile Technology Control Regime. China also backed United Nations sanctions that imposed a broader arms embargo on Iran.

The U.S. State Department has called the leaks illegal. U.S. officials declined Monday to discuss the contents of the cables or how they might affect U.S. foreign policy. Mrs. Clinton spoke about the leaks by telephone with her Chinese counterpart, Yang Jiechi, on Friday, the State Department said. China's Foreign Ministry didn't respond to a request to comment Monday.

The publication of the cables comes at a sensitive time in China-U.S. ties, as Beijing faces mounting pressure from Washington to rein in an increasingly belligerent North Korea ahead of President Hu Jintao's U.S. visit in January.

U.S. and European officials, along with counterproliferation experts, have increasingly in recent months focused on Beijing's continuing, and possibly intensifying role in aiding Iran's nuclear and missile programs.

These officials note that China's role in Iran's procurement activities appears to be growing, as Tehran's other trading partners, including the United Arab Emirates and Germany, have moved to comply with U.N. sanctions and stifle commerce with Iran.

In recent months, U.S. and allied law-enforcement agencies have exposed a number of cases where Chinese-based firms have sought to sell dual-use equipment to Iranian entities linked to Tehran's weapons programs.

An analysis of the Iranian missile threat last month by Arms Control Today, published by the independent Arms Control Association in Washington, suggested U.S. pressure on Beijing has produced only mixed results. The content of the leaks "shows either China's inability to enforce its own export laws, or a kind of malign negligence," said Peter Crail, a research analyst at the ACA who covers North Korea. "There's a pattern of frustration on the part of the U.S. government."

Mr. Crail said one factor could be China's continuing support for the North Korean regime, which earns much of its hard currency from exports of missile technology, often sold through front companies based in China.

Last year, New York financial regulators disrupted an attempt by a Beijing-based company to sell 66,000 pounds of tungsten copper used in missile-guidance systems to an Iranian company. Earlier this year, The Wall Street Journal reported in how an Iranian firm purchased special hardware for enriching uranium from a Chinese company based in Shanghai. The Chinese company had originally purchased the equipment from a French firm previously owned by the U.S. industrial conglomerate, Tyco International.

Nonproliferation experts said Chinese firms are specifically playing this role as a procurement agent on behalf of Iranian entities. In many cases, these Chinese firms aren't capable of producing the sophisticated metals or machinery themselves. But they identify themselves as the end-users of the products, thus helping Tehran evade a widening global sanctions regime.

"It's this third-party role that China's playing which is the most dangerous," said Paul Brannan, also of the Institute for Science and International Security.

China's role in Iran's nuclear and missile programs has increasingly caught the eye of Congress. U.S. lawmakers, including the soon-to-be chairwoman of the House Foreign Affairs Committee, Ileana Ros-Lehtinen (R., Fla.) have pledged to press the White House to more vigorously implement sanctions against Chinese firms abetting Iran's weapons procurement. Obama administration officials said they have passed on to Beijing in recent months a list of Chinese companies the U.S. believes are violating new U.S. legislation.

The cables also highlight U.S. concerns about China's computer-warfare capability, and its influence in Central Asia. And they give potentially embarrassing blow-by-blow accounts of U.S. diplomats' meetings.

One cable from the U.S. Embassy in Beijing quoted an unidentified Chinese contact alleging in January this year that the Politburo, the powerful 25-person governing group in the Communist Party, ordered a cyberattack on Google Inc. as well as U.S. government computer systems.

A Google spokeswoman said: "We have conclusive evidence that the attack came from China." She declined to elaborate. China's government has repeatedly denied any involvement in any cyberattacks.

Another cable described how the U.S. ambassador in Kyrgyzstan confronted her Chinese counterpart, Zhang Yannian, over information obtained from Kyrgyz officials that China offered the former Soviet republic $3 billion in exchange for its closing a U.S. air base there.

"Visibly flustered, Zhang temporarily lost the ability to speak Russian and began spluttering in Chinese," said the cable, adding that Mr. Zhang later composed himself, and "ridiculed" the idea without categorically denying it.

Mr. Zhang, now China's ambassador to Azerbaijan, couldn't be reached for comment.

Another cable that could complicate U.S. diplomacy in Beijing gave a detailed account of a conversation between a U.S. political officer and Li Guofu, an expert on the Middle East at the China Institute for International Studies, which is affiliated with the Foreign Ministry.

That cable said Mr. Li suggested that the U.S. negotiate a secret deal with Iran, allowing it limited uranium-enrichment operations in exchange for closer international supervision and a suspension of its support for Hamas and Hezbollah.

"Some of the talks between me and my diplomat friends are not supposed to be open for public," Prof. Li said in an interview. He said he will be more careful in future talks with U.S. diplomats.

The most serious allegation in the cables is that China repeatedly turned a blind eye to shipments of missile components through Beijing on commercial flights operated by Air Iran, the Iranian national carrier, and Air Koryo, the North Korean one.

It is unclear whether China complied, but the cable complained that at least 10 similar deliveries had been allowed to proceed despite U.S. requests for them to be halted.

Three cables sent by Mrs. Clinton in February this year show the U.S. still had concerns about Iran obtaining missile technology from China.

The cable said the State Department sought "immediate action," and instructed the U.S. ambassador in Beijing to raise the issue "at the earliest opportunity" and "at the highest level possible" to persuade the Chinese authorities to halt the delivery.

One instructed U.S. diplomats to ask Chinese officials to act on intelligence that Iran was trying to buy Russian gyroscopes, which can help to stabilize and guide ballistic missiles, from a Chinese company.

A second cable said Iran was trying to buy the same gyroscopes from China through a Malaysian company, and a third said Tehran was seeking to purchase five tons of carbon fiber—which could be used to make nozzles and casing for its missiles—from a Chinese company.

Another cable from Mrs. Clinton in May said the U.S. was concerned that exports by named Chinese companies "could be used for or diverted to a CW [chemical weapons] program" and asked if the transfers were approved by the Chinese government.