In Karbala, Iraq, a sign showing the anti-American sentiment.

WASHINGTON,  — Confronting mounting resistance in Iraq from militant Islamic clerics and Arab nationalists, the White House said today that it was determined to see an "Islamic democracy" built in Iraq and that it had recently warned Iran against interfering with its efforts to organize a government. The warning to Iran reflected a growing concern in the White House that the American-led plans for the occupation of Iraq were facing fresh difficulties. Responding to the instability still evident in Baghdad, competition for power among Kurds and Arabs in the north and an anti-American outpouring from Shiites in the south, a senior administration official said today that "it's clear we are going to have to step in a little more forcefully."  American officials said the United States was redoubling its efforts to promote pro-American clerics and other leaders in Iraq to counter the radical messages being heard from many mosques, particularly in the heavily Shiite south.

As part of the effort, Special Forces troops and intelligence officers are identifying friendly clerics in small towns and cities and encouraging them to issue religious edicts, or fatwas, in support of the postwar American administration.

The opposition to the American presence has been most vocal from Shiite Muslims, who represent the majority of Iraq's population, but it has also come from Sunni Muslims in places like Baghdad, who have in some cases joined forces with Shiites in calling for an early end to occupation. In Mosul, home to tense rivalries between Iraqi Kurds and Arabs, the opposition to the American forces has made for the country's sharpest tensions, as American troops having clashed with angry Iraqi civilians.

Officials said the warning had been communicated through third parties to the Tehran government in recent days. Government officials have said that Iranian agents have crossed into Iraq since the fall of Saddam Hussein and are trying to court Shiite clerics sympathetic to Iran's fundamentalist government.

The White House spokesman, Ari Fleischer, said today that "there is no love lost between the Iraqi people and the Iranian people," noting their long history of conflict. He told reporters that the United States sought something closer to the model of Islamic democracy in Turkey, and added that the United States would oppose any effort by states that support terrorists — he specifically named Iran and Syria — to mold Iraq's future government.

"We've made clear to Iran that we would oppose any outside interference in Iraq's road to democracy," he said. "Infiltration of agents to destabilize the Shiite population would clearly fall into that category."

In making the public statements today, administration officials are clearly hoping to force Iran's leaders to back off, much as Syria did — at least temporarily — last week. Yet Iran, the officials noted, is a far more complex political problem, and internally some administration officials have warned that the American presence in Iraq could spur Iran to speed up its nuclear weapons program.

The White House today went to considerable lengths to point up the national and ethnic differences that officials said should limit Iran's influence in Iraq. "I want to stress that people should not overinterpret the capability of the Shia Iranians to influence the Shia Iraqis," Mr. Fleischer said. "They are not one and the same." 

Administration officials have been surprised by the ferocity of the anti-American sentiments being voiced in some quarters of Iraq, and some said today that the hostile mood was prompting some rethinking of how best to build the kind of friendly, democratic government that the United States has said it hopes will eventually emerge in Iraq. 

"We're flying a little blind here," one official said, adding that in retrospect the administration had probably put too much faith in assurances from Ahmad Chalabi, a secular Shiite who is a leader of the Iraqi National Congress, that the vast majority of his countrymen would welcome not only the American invasion but also American-led efforts to shape a new Iraq.

There were signs today that the plan for handing over power to an "interim Iraqi authority" within the next six months might be in flux, as officials dealt with the power struggles now under way from Mosul in the north to Basra in the south.

The various forces who have challenged the American-led administration include Arab nationalists in Mosul, who look with suspicion on the close ties between the Untied States and the region's Kurds, and two returned Iraqi exiles in Baghdad, whose claims of authority in the capital are being rejected by American officials. 

Before the war, American officials said the United States anticipated that there would be efforts by neighboring states to influence the creation of a new government, and perhaps to undermine the American presence. But the original plan may have underestimated the degree of anti-American opposition, and the speed with which the Shiite clerics would organize themselves into a political force.

"There are parts of our post-Saddam plan," one senior official said today, "that we know will have to get rewritten to fit the facts on the ground."

A team of administration officials led by Zalmay Khalizad, a National Security Council official, is heading to Baghdad to convene a second meeting of Iraqi leaders on Saturday, a step toward the formation of the interim Iraqi authority. Behind the scenes, however, there is debate within the administration as to how aggressively the United States should be moving to assert influence and to court supporters in a power vacuum in which some of the loudest voices so far have been anti-American, and where Iran has shown signs of trying to play an active role.

One senior Pentagon official described the American role so far as "keeping a light hand on the tiller." But while the United States has publicly described its role as promoting a stable, secure environment in Iraq that might be conducive to democracy, administration officials said they also recognize the need to move more aggressively to promote allies.

"You can't stand back completely," the Pentagon official said.

A United States government official said the Central Intelligence Agency and other arms of the government were actively involved in courting a network of supporters to extend far beyond Mr. Chalabi, a Pentagon favorite who is viewed with deep suspicion at the State Department and other government agencies. "It's sensitive, because if we talk about what we're doing, it could rebound against the people we're trying to help," the official said. "But we're not letting the vacuum go unfilled."

In a briefing for reporters today, the commander of American ground forces in Iraq, Lt. Gen. David McKiernan, said: "Right now the Shia and any Iranian-influenced Shia actions are not an overt threat to coalition forces. But we're watching all these competing interests. And if truth be known, this is probably a little bit of democracy in process right now here in Iraq."

One Defense Department official in Washington said the administration expected the signs of dissent to diminish over time.

"I don't think we should be too concerned that we're seeing people who say, `Saddam's gone and we want the Americans gone, too,' " the official said. "If all you've ever known is the brutality that a uniform represents, from the Iraqi regime, it's not going to be any different, at least at first, when you seen an American uniform."