Bush administration officials said yesterday they are pleased by Iran's willingness to cooperate with U.S. requests during the war with neighboring Iraq -- a decision perhaps smoothed by the administration's bombing of Iranian opposition fighters based in Iraq. But it is unclear if this "good behavior" signals a thaw in relations with the Islamic republic.

The administration "has been very clear with the Iranians about the kind of behavior we expect," one official said. Iran sealed its border and prevented senior Iraqi officials from fleeing. Iran also did not protest too hard when U.S. munitions accidentally fell on Iranian soil.

Iran cooperated during the war with Afghanistan, another neighbor, but relations quickly chilled and Iran became a charter member of President Bush's "axis of evil," along with Iraq and North Korea. But several officials said yesterday that for the moment, Iran has been eclipsed as a potential problem by Syria, which appears to have actively aided Iraq during the war.

"They have behaved rather well," another senior U.S. official said of Iran. Syria, by contrast, "established tactical cooperation with Iraq, almost as if they calculated this war would go differently. And they bet wrong."

Yet the official said the administration has not formally considered what this cooperation means for U.S.-Iranian relations. "It is one of the issues that will be looked at in the future."

Two senior U.S. officials -- Zalmay Khalilzad from the White House and Ryan C. Crocker from the State Department -- met secretly in January with Iranian officials to discuss potential cooperation. The U.S. officials asked that Iran seal its border to prevent the escape of Iraqi officials, among other requests, and suggested that the United States would target the Iraq-based camps of the Mujaheddin-e Khalq Organization, or People's Mujaheddin, a U.S. official said.

"We told them they would find it advantageous" if the United States struck the Mujaheddin camps, the official said. A more concrete commitment to attack the camps was later relayed to Tehran through British officials. The Mujaheddin-e Khalq, which has been a source of information on Iran's nuclear programs, has protested angrily about the attacks, saying they were unprovoked.

The group says thousands of Iranian troops have entered Iraq. But U.S. officials believe there have been only limited incursions by Iranian troops, mostly as mopping-up operations against the Mujaheddin-e Khalq.

The Bush administration has long waged an internal debate over relations with Iran. State Department officials have pressed for a greater effort at engagement, and the Pentagon and White House have urged a tough line against a country that some officials believe is on the verge of revolution.

Bush seemed to end the argument last July when he issued a statement declaring his solidarity with protesters in the street. "The vast majority of the Iranian people voted for political and economic reform," Bush said. "Yet their voices are not being listened to by the unelected people who are the real rulers of Iran."

That sentiment has been publicly echoed by other senior officials.

CIA Director George J. Tenet has been remarkably outspoken in his view that Iran's split government has to end. In open testimony last February before the Senate intelligence committee, he called the current leadership "secure, but increasingly fragile," and blamed its condition on "the reluctance of reformist leaders to take their demands for change to the street."

He said the United States was looking for new Iranian leadership but was "unable to identify a leader, organization, or issue capable of uniting the widespread desire for change into a coherent political movement that could challenge the regime," which the director noted had "publicly argued in favor of using deadly force if necessary to crush the popular demand for greater freedom."

Administration officials believe Iranian foreign policy is controlled by the mullahs who direct the nation's national security apparatus. As Tenet put it, "Conservatives already control the more aggressive aspects of Iranian foreign policy, such as sponsoring violent opposition to Middle East peace." Along with Syria, Iran continues to be one of the few open state sponsors of terrorism, although at a lower level than previously. The nation is the prime supporter of Lebanese Hezbollah, and with Syria gives aid to the Islamic Resistance Movement, or Hamas, and the Palestine Islamic Jihad. The three are the most active of the terrorist groups fighting for Palestinian goals against Israel.

Tehran also has alarmed officials by proceeding rapidly with its nuclear weapons program, which has the support both of the mullahs and the reformers. Despite repeated protests by Washington and Iran's participation in the nuclear Non-Proliferation Treaty, "the United States remains convinced Tehran is pursuing a nuclear weapons program," according to the declassified version of a CIA study that was released last week.

Nevertheless, some U.S. officials see signs of hope. Over the weekend, Iran's former president, Ali Akbar Hashemi Rafsanjani, was quoted as saying that Iran's resumption of ties with the United States could be put to a referendum. In an apparent reference to previous failures by the countries to begin a constructive dialogue, he said: "We missed certain opportunities, or took late or wrong measures, or even did not take action."

Rafsanjani's suggestion was dismissed by Iran's president, Mohammad Khatami, but one U.S. official described Rafsanjani's statement as a "very significant development."

U.S. officials denied they targeted the Mujaheddin-e Khalq as a way to curry favor with the Iranians, describing it merely as a "confluence of interests." The group, which U.S. officials said is the most serious security threat to the Iranian government, has maintained thousands of fighters armed with tanks, armored vehicles and artillery along the Iraq-Iran border for the past decade.

While many in the U.S. administration share the group's antipathy for Iran's radical Islamic government, the State Department has labeled the group as a terrorist organization for its history of attacks on both Western and Iranian targets. U.S. analysts have concluded that its primary support has come from the Iraqi government, despite some financial backing from Iranian expatriates elsewhere in the world. One U.S. official said yesterday some mujaheddin fighters joined in attacking U.S. troops.

Mohammad Mohaddessin, a senior official with the group's political arm, the National Council of Resistance of Iran, denied the allegations in an interview from Paris. He said he informed U.S. officials in February that the group had vacated most of its camps to avoid being drawn into the conflict. "We have not fired a single bullet throughout the course of this war," he said. The bombing strikes, which caused casualties, are "truly astounding and regrettable," he said.

Correspondent Alan Sipress contributed to this report from Doha, Qatar.

This commercial satellite photograph shows the construction site for a nuclear facility at Natanz, Iran. A CIA study said Iran is pursuing a nuclear weapons program.