The United States has imposed sanctions on five Chinese firms and a North Korean company for selling weapons technology to Iran.
The US says Iran is seeking nuclear weapons
The State Department spokesman, Richard Boucher, said the technology could have helped Iran to develop weapons of mass destruction.
The penalties include a ban on trading with, and receiving assistance from, the US Government.
Four out of five of the companies have previously been sanctioned for engaging in similar technology transfers to Iran and Pakistan.
It is not known exactly what kind of technology the companies sold to Iran in the latest case.
The Chinese companies were named as: Taian Foreign Trade General Corporation of China; the Zibo Chemical Equipment Plant of China; the Liyang Yunlong Chemical Equipment Group Company of China; China North Industries Corporation (Norinco) and the China Precision Machinery Import/Export Corporation (CPMIEC).
China's Foreign Ministry denied the charges. "We've expressed our strong disatisfaction and stern opposition to the US side," a statement said.
The North Korean firm was named as the Changgwang Sinyong Corporation.
State Department spokesman Richard Boucher said the penalties were imposed "for the transfer to Iran in the first half of 2002 of equipment and technology listed on
multilateral export control lists, or otherwise having the potential to make a material contribution to weapons of mass destruction or missiles".
The firms were punished under the 2000 Iran Non-Proliferation Act.
Taian Foreign Trade General Corporation of China
Zibo Chemical Equipment Plant of China
Liyang Yunlong Chemical Equipment Group Company of Chin
China North Industries Corporation (Norinco)
China Precision Machinery Import/Export Corporation (CPMIEC
Changgwang Sinyong Corporation (North Korea)
America has accused Iran of seeking to develop nuclear weapons, a charge the Islamic republic denies.
The sanctions will remain in place for two years.
Correspondents say most of the companies involved will not be much affected, as all but the Taian Foreign Trade General Corporation of China were already under penalties in the US and did not, in any case, do much business there.