U.S. soldiers seized the men in a raid on an Iranian government office in the northern Iraqi city of Arbil on Jan. 11, hours after U.S. President George W. Bush pledged to crack down on the "flow of support" from Iran to Iraqi militants.

Washington regularly accuses Iran of encouraging violence in Iraq, a charge Tehran dismisses saying the U.S. occupiers are destabilising the country. Iran has demanded the immediate release of the Iranians.

"Soon we will complain to the Security Council and the United Nations for this adventurism by America," Foreign Minister Manouchehr Mottaki said on state television. He did not say when Iran would take such a step. "Instead of securing Iraq, America is following heathen aims," he said, adding that the United States wanted "to make regional countries face off with Iran".

Tehran accuses Washington of trying to line up nearby Arab countries, who are dominated by Sunni Muslims, against Iran, which like Iraq has a Shi'ite Muslim majority.

The arrest on Jan. 11, the second such detention in a month, have heightened tensions between Washington and Tehran, already at loggerheads over Iran's nuclear programme which U.S. officials say is aimed at making atomic bombs.

Iran insists its aims are civilian and part of plans to generate electricity.