US lawmakers united Wednesday in anger over China's links with Iran, but also blamed the administration of Barack Obama for not imposing sanctions on Chinese companies trading with Tehran.

The harsh criticism came from both Republicans and Democrats after leaked secret cables show US frustration in trying to get China to do more to stop the proliferation of missile technology.

A cable from 2007 cited by the British newspaper the Guardian disclosed that China was given specific details about a missile parts shipment that was expected to transit Beijing.

US diplomats were instructed to express US concern "at the highest level possible."

Other cables detailed what US diplomats said was the delivery to Iran of 19 North Korean missiles with a range of up to 2,000 miles (3,000) kilometers.

"There seems to be no doubt that Chinese companies are pursuing energy investments and selling Iran refined petroleum. The Chinese acknowledge it," said Howard Berman, the Democratic chairman of the House Foreign Affairs Committee.

"I'd like to know why we haven't sanctioned any of the Chinese companies engaged in clearly sanctionable actions," he told the committee gathered to assess the impact of tightened sanctions on Iran.

Sanctions czar Stuart Levey and Undersecretary of State William Burns -- the top negotiator heading to international talks in Geneva next week over Iran's nuclear program -- appeared before lawmakers.

The powerful panel's top Republican, Ileana Ros-Lehtinen, directly accused China of "eagerly expanding trade" with the Islamic republic and of being a "key ally and protector of Iran that has made clear that it will prevent significant pressure on Tehran."

She said China's purported help in facilitating North Korea's shipments of advance missiles and chemical weapons ingredients to Iran was a direct violation of UN Security Council sanctions resolutions.

Several lawmakers warned that Chinese firms would eagerly "backfill" the investment gaps left by Western firms pulling out of Iran if the robust new sanctions regime was not imposed in an even-handed manner.

"China attacks us in a hundred ways," including on the economic and national security fronts, said Democratic Representative Brad Sherman, who has proposed revoking Beijing's most-favored nation trading status.

The Asian giant "subsidizes rogue regimes" and threatens to veto resolutions at the UN Security Council to protect Iran, he told the committee.

"And I don't blame so much Beijing as I blame Washington," he said, noting that the administration has hit China only with harsh talk, "in the hopes that our words will sting so badly that Beijing will change its policies."

Representative Ed Royce urged the White House to get tough with Beijing over its links to Iran.

"This conduct on the part of China in terms of violating these sanctions and helping give Iran the wherewithal to develop the missile technology and the nuclear weaponry has to stop immediately," the California Republican said.

"And if it does not stop, there certainly is going to be legislation from this Congress to bring it to a halt."

Burns, the senior diplomat, insisted President Barack Obama's administration was doing its best to lean on Beijing.

"We take seriously every piece of information that we see. We have on a number of occasions raised that information with the Chinese government," he said.

"In some cases, we're seeing them act on it. But the record is a mixed one, to be honest, and we're continuing to press those specific concerns that we have."