BAGHDAD, Iraq -- With a new Iraqi government in place, Iran is positioning itself to play a major role here at a time when American influence is showing signs of faltering.

That is worrisome to Iraq's Arab neighbors, especially Sunni-dominated countries such as Saudi Arabia and Egypt. But it also raises serious questions for Washington, including the wisdom of withdrawing entirely from Iraq when it has long been considered the eastern defense against Iranian expansion.

Concerns about Iran have simmered since the March 2003 U.S.-led invasion of Iraq removed a Sunni-dominated dictatorship and set the stage for democracy -- or, inevitably, Shiite rule in a country where Shiites hold an overwhelming majority.

Those issues have now come to the fore because of Iran's confrontational stance over its nuclear program. In effect, Iran's recent robust behavior in Iraq serves to remind Washington that it has its own cards to play -- including influence among Iraqi Shiites -- if the Americans threaten Tehran militarily over its plans to enrich uranium.

Iran has wasted little time in moving to shore up ties with the new government that took power last month in Baghdad. Iranian Foreign Minister Manouchehr Mottaki flew to Baghdad on Friday, where he was warmly welcomed by Iraq's new leadership, including not only Shiites but also Sunni and Kurdish politicians.

In addition, work has already started on a multimillion-dollar international airport near the Shiite holy city of Najaf, financed mostly by a low-interest loan from Iran. The airport is designed to serve Shiite religious pilgrims visiting Najaf's shrines and provide a major boost to the economy of Iraqi's impoverished Shiite south.

All that is alarming to the Middle East's majority Sunni Arab governments -- including Jordan, Egypt and Saudi Arabia -- which have long feared Iranian influence among Shiites throughout the Persian Gulf because it could undermine pro-American regimes in the oil-rich region.

Last year, Jordan's King Abdullah II warned that Iran wants to create "a Shiite crescent" that would disrupt the balance of power in the region. The Saudi foreign minister gave similar warnings.

Egyptian President Hosni Mubarak then outraged Shiites across the region this year when he said that Iraqi Shiites were more loyal to Iran than to their own country.

Arab governments fear a bleak future in which Iraq either descends into civil war or ends up closely allied with Iran.

As long as substantial U.S. forces remain in Iraq, those dark scenarios seem unlikely. But prospects for a long-term U.S. presence here look shaky as American public opposition to the war grows.

If the Americans leave, the Iranians are waiting in the wings.

It is unlikely, however, that Iran would take a high-profile role, given the ambivalent feelings that most Iraqis, even Shiites, hold toward their eastern neighbor after a bitter 1980s war that killed an estimated 2 million people.

Sunni Arabs deeply distrust Iranians and consider Shiite politicians little more than Iranian agents. Rumors circulate widely that Iranian intelligence agents direct death squads in Baghdad.

"Iran wants to shape the situation in its favor, but does not want to be, or be perceived as, heavy-handed about it," said Juan Cole, an expert on Shiite Islam at the University of Michigan.

Instead, the Iranians prefer to work behind the scenes, doling out cash to key Shiite political players -- chief among them the biggest Shiite political party, the Supreme Council for the Islamic evolution in Iraq.

Iran's hand also is rumored to be behind Shiite militias in Basra, although little evidence of a direct link has been made public.

Nevertheless, U.S. officials have long accused the Iranians -- though not necessarily the Tehran government-- of smuggling weapons to Shiite militias in Basra and perhaps also selling roadside bomb technology to Sunni militants -- charges Iran denies.

If the charges are valid, it may be that Iran wants to keep Iraq bubbling just enough to tie down the Americans and keep them from any military moves against Tehran.

Ironically, both the United States and Iran share an interest in preventing Iraq from disintegrating into full-scale civil war, something that would threaten Shiite political power in Iraq and risk angering Iran's Arab minority.

But the nuclear standoff, as well as a generation of bitterness, has prevented the two countries from working together. U.S. Ambassador Zalmay Khalilzad has offered to talk to the Iranians but only about the situation in Iraq.

Mottaki, the Iranian foreign minister, ruled out talks with the Americans, citing unspecified conditions. That suggested the Iranians want to hold out until Washington is ready to put everything -- including the nuclear issue -- on the table.

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Robert H. Reid is correspondent at large for The Associated Press and has reported frequently from Iraq since 2003.