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 more than a quarter-million diplomatic cables the website WikiLeaks began making public Sunday.

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

The cables 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, that could help Iranian weapons programs. 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.”