"We have well-known channels of communication with Iran and we have made clear to Iran that we would oppose any outside interference in Iraq's road to democracy," said White House spokesman Ari Fleischer.

"Infiltration of agents to destabilize the Shi'a population would clearly fall into that category," Fleischer said, amid media reports that Tehran had dispatched agents to shore up its interests in Iraq.

Fleischer would not explicitly confirm the reports, and dodged repeated questions on the precise nature of Washington's message to Tehran.

The rebuke, coming shortly after Saddam Hussein's ouster, recalled a similar warning in the aftermath of the US-led campaign in Afghanistan, when Washington cautioned Iran not to seek to expand its influence in eastern areas.

Earlier, The New York Times, citing US officials, reported that Iran-trained agents were crossing into southern Iraq to promote friendly Shiite clerics and possibly an Iranian model of Islamic government.

The unnamed officials told the daily that, based on intelligence reports, some agents were members of the Badr Brigade, the military wing of an Iraqi exile group operating from Iran, and irregular members of a special unit of Iran's Revolutionary Guards.

"They are not looking to promote a democratic agenda," one military official told the Times.

Officials told the Times that the Shiite Muslim pilgrimage to Iraq's holiest city of Karbala, 80 kilometers (50 miles) to the south of Baghdad, provided cover for the Iranian agents, and that members of the Badr Brigade have been seen to "shed their uniforms, put on civilian clothes, and disappear."

Hundreds of thousands of Shiite Muslims flocked to Karbala Wednesday, but large anti-US protests failed to ignite despite appeals by clerics for mass shows of anti-Americanism.

Fleischer downplayed concerns that Iran would win a warm welcome in Iraq, even though both nations have a predominantly Shiite Muslim population, because those populations are very different.

"I think that any efforts or anybody outside of Iraq to try to create an outsider's version of what should take place for the Iraqi people, by the Iraqi people, will not have much chance of success," said Fleischer.

The United States severed formal diplomatic ties with Iran after the 1979 revolution and the bitter embassy hostage crisis.

Washington routs most of its contacts with Tehran through the Swiss diplomatic mission in the Iranian capital as well as through a Geneva-based, UN-sponsored forum created to discuss Afghanistan.

US President George W. Bush has lumped Iran in with Iraq and North Korea in an "axis of evil." Lately, however, his public warnings have been aimed more at Syria, which he accused of harboring remnants of Saddam Hussein's regime.

US officials, speaking on condition of anonymity, said that Tehran may be seeking to promote an Iranian model of government or at least building bases of support for Iran.

Iraq's majority Shiite population was oppressed for years under Saddam's dictatorship. After the 1991 Gulf War, the US-led coalition, it is believed, did not try to overthrow Saddam for fear that a Shiite-led Islamic goverment would take his place.

Fleischer strove to differentiate between US opposition to "an Islamic dictatorship" and a new Iraqi government run by Muslims.

"I think it's a given that it will be an Islamic leader, it's an Islamic country."

"That's different from an Islamic dictatorship that doesn't respect the religious disagreements among the people, that is not tolerant, that is dictatorial, that is closed, that doesn't govern by a rule of law or transparency," he added.

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