Beijing is reluctant to launch a full crackdown on Chinese firms conducting business in violation of UN Security Council sanctions, the Pentagon's Asia chief said Wednesday.

"The US government has asked Beijing to halt commercial transactions by Chinese firms that violate UN sanctions, nonproliferation norms, and PRC (China's) law but our efforts are met with mixed results," said James Shinn, assistant secretary of defense for Asian and Pacific security affairs.

"China's willingness to cooperate on these is uneven," he said at a hearing on security developments by the US House of Representatives Armed Services Committee.

Shinn was particularly concerned over China's sale of conventional weapons to Iran, accusing the Islamic republic of supporting militant groups in Iraq, Lebanon and Afghanistan "that target and kill Americans and our allies."

The United States and allies have also accused Iran of developing nuclear weapons but Tehran says its atomic program is for peaceful purposes.

However, Iran is defying UN demands that it stop uranium enrichment, a technology that could be used to make nuclear arms.

UN Security Council sanctions block the sale to Iran of equipment and technology related to nuclear activities and also of so-called dual-use items, which can have either a military or civilian purpose.

"We look to China to act responsibly and restrict conventional arms sales that promote instability and violate international norms," Shinn said.

Similarly, Shinn said, the Pentagon "still observe Chinese firms and individuals transferring a wide variety of weapons-related materials and technologies to customers around the world" -- including to Myanmar, Zimbabwe, Cuba, Sudan and Syria.

In April, a ship belonging to a Chinese state-owned shipping firm was forced to abandon plans to deliver a shipment of arms to Zimbabwe amid fears they could be used to crack down on opposition supporters.

China is a major supporter of beleaguered Zimbabwean leader Robert Mugabe.

Shinn also said that the United States was "troubled" by China's lack of openness and transparency in military affairs.

"This opacity raises questions as to China's true intentions and compels outside observers to compare China's behavior and capabilities against its declaratory policies," he said.

Shinn said the United States looked forward to China's upcoming submission of its defense expenditures to the United Nations, saying it had so far significantly underreported its military budget.

China's announced defense budget for 2007 was 45 billion dollars and 58 billion dollars in 2008 but the Pentagon said it did not include expenditures for big items such as foreign acquisitions, expenses for strategic forces and military-related research and development.

The Pentagon estimates China's total military expenditures in 2007 to be between 97 and 139 billion dollars.