The companies will be banned from exporting goods to US government agencies or receiving assistance from the United States and US firms will be barred from doing business with them for two years, spokesman Adam Ereli told reporters.

Ereli said the sanctions were being imposed under the Iran Nonproliferation Act of 2000 and would be in effect until March 31, 2006.

A top US official last month accused Iran of concealing a nuclear weapons program and vowed to maintain international pressure on Tehran to reveal its efforts.

The 13 companies include five Chinese companies, two in Macedonia, two in Russia and one each in North Korea, Taiwan, Belarus and the United Arab Emirates, Ereli said, noting the sanctions affected only the specific companies, not their respective governments or countries.

"The penalties were imposed ... because there was credible information indicating that these companies had transferred to Iran, since January 1, 1999, either equipment and technology on the ... multilateral export control lists" or items that could make a "material contribution" to weapons of mass destruction or cruise or ballistic missile systems, he said.

Ereli said the United States had imposed sanctions on a total of 23 entities since the law took effect, including four companies sanctioned last year.