But in Washington, a Pentagon spokesman denied that the building was a consulate and insisted the captives are "suspected of being closely tied to activities targeting Iraq and coalition forces."

"We don't know the reason for this," an Iranian diplomat in Baghdad told AFP on condition of anonymity. "The Americans arrested five employees and took all the computers and documentation." The consulate building in Arbil was sealed off by Kurdish security forces, and local officials confirmed that there had been arrests by US forces.

The raid came just hours after US President George W. Bush announced he had ordered 21,500 more troops to Iraq and promised to take aggressive steps to curtail what he described as Iranian and Syrian help for insurgents.

Speaking on television, US Secretary of State Condoleezza Rice warned Washington was determined to "pursue" Iranians trying to disrupt US efforts to stabilise Iraq and crack down on Iran's "regional aggression".

"You will see that the United States is not going to simply stand idly by and let these activities continue," she said on Fox News.

Tehran angrily condemned the operation and accused arch enemy Washington of violating international law and stirring up tension among Iraq's neighbours.

"This is a provocative US action and it is contrary to all international laws, and it is condemned," foreign ministry spokesman Mohammad Ali Hosseini told state radio.

"By doing this the Americans are following two aims, firstly to continue their pressure on Iran and secondly to create tension among Iraq's neighbours," he said, adding that Iran had asked Iraqi officials to pursue the case.

The Kurdish regional government in northern Iraq -- normally a staunch supporter of US policy in the region -- also condemned the raid and demanded the immediate release of all prisoners taken.

"The presidency and the government of the Kurdish region of Iraq express their disapproval of the operation against the Iranian consulate," said a statement from regional president Massud Barzani's office.

The statement recalled that diplomatic premises were protected from attack under international protocols and accused US forces of "damaging efforts to restore stability and security in Iraq."

Pentagon spokesman Bryan Whitman said: "I can confirm for you through our forces there that this is not a consulate or a government building." Iraqi government spokesman Ali al-Dabbagh told reporters that Baghdad had asked the Americans to clarify who was arrested and why, confirming that the foreign ministry in Baghdad was investigating the matter.

"We need to identify who those people are and why they were arrested. We have an office in Arbil and the foreign ministry in Baghdad is working on this to find out exactly what happened," he said.

Washington has not confirmed that diplomats were targeted, but has long accused Tehran's agents of fomenting unrest in Iraq and smuggling weapons to militias involved in sectarian violence and attacks on US forces.

"Coalition forces conducted routine security operations in northern Iraq and took six individuals into custody suspected of being closely tied to activities targeting Iraqi and coalition forces," the US military in Iraq said.

Egyptian President Hosni Mubarak, who is expected to meet Rice during an imminent tour of the Middle East, stood shoulder to shoulder with his US ally, warning Shiite Iran: "Don't touch Iraq," in a newspaper interview.

Last month, US forces detained two Iranians among eight people rounded up on suspicion of weapons smuggling in Baghdad. They were later released.

Tension between Iran and the United States has soared after the UN Security Council voted to impose sanctions over Iran's nuclear programme and Tehran vowed to start immediately expanding its capacity to enrich uranium.

Washington accuses Iran of seeking to develop a nuclear weapon, a charge vehemently denied by the oil-rich Islamic republic, which says it only wants to provide atomic energy to a growing population.

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