WASHINGTON — The Obama administration is determined to press on with efforts to engage the Iranian government, senior officials said Saturday, despite misgivings about irregularities in the re-election of President Mahmoud Ahmadinejad. The White House’s cautious reaction reflected the combustible scene in Tehran, where riot police officers were cracking down on opposition supporters, and the likelihood that the administration would be forced to pursue its diplomatic initiative with a familiar and implacable foe, who now also has a legitimacy problem.  “We, like the rest of the world, are waiting and watching to see what the Iranian people decide,” Secretary of State Hillary Rodham Clinton said during a visit to Niagara Falls on Saturday. “We obviously hope the outcome reflects the genuine will and desire of the Iranian people.” There was palpable disappointment within the administration, where there were hopes, as President Obama said on Friday, that the throngs of people at the polls augured a change in Iran.

Trying to put a positive face on the outcome, one senior administration official held out the hope that the intensity of the political debate during the campaign, and the huge turnout, might make Mr. Ahmadinejad more receptive to the United States, if only to defuse a potential backlash from the disputed election. “Ahmadinejad could feel that because of public pressure, he wants to reduce Iran’s isolation,” said the official, who spoke on condition of anonymity because the delicacy of the matter. “That might also cause engagement to proceed more swiftly.” But outside analysts said the suspicions surrounding Mr. Ahmadinejad’s re-election would create new problems. “This is the worst result,” said Thomas R. Pickering, a former under secretary of state. “The U.S. will have to worry about being perceived as pandering to a president whose legitimacy is in question. It clearly makes the notion of providing incentives quite unappetizing.” Mr. Pickering, who has had informal contacts with Iranians, said the White House would have little choice but to accept the results. But he said the outcome would hinder efforts to court Tehran and would embolden those who argue that such efforts are futile.

Many analysts and Middle East officials asserted that the outcome reinforced the reality that ultimate power resides not in the democratically elected president, but rather in Iran’s supreme religious leader, Ayatollah Ali Khamenei. “We should be clear about what we’re dealing with,” said Karim Sadjadpour, an Iran expert at the Carnegie Endowment for International Peace. “Just as we deal with Assad’s Syria and Mubarak’s Egypt, we now have to deal with Khamenei’s Iran,” he said, referring to Bashar al-Assad and Hosni Mubarak. In Israel, which has hinted that it might launch a military strike on Iran to disable its nuclear capability, officials said Mr. Ahmadinejad’s victory underscored the threat from Tehran and the need for a tough response rather than patient diplomacy. Vice Prime Minister Silvan Shalom told an audience in Tel Aviv on Saturday that the Ahmadinejad victory “sends a clear message to the world” that Iran’s current policies have broad internal support and will be continued. The results, he added, also “blow up in the faces of those who thought Iran was built for a genuine dialogue with the free world on stopping its nuclear program.”  In the Arab world, reaction was largely split between those aligned with Iran and rely on Mr. Ahmadinejad’s aggressive posture to the West — like the militant group Hezbollah — and American allies, which have felt threatened and bullied since he came into office. Arab diplomats and political analysts said that they did not believe that Iran would change its nuclear policy regardless of who the president was.

“It is easy to insult and confront and have Iran as a foe when Ahmadinejad is president,” said an Egyptian diplomat, speaking on condition of anonymity in keeping with diplomatic protocol. “A lot of people would have been inconvenienced if someone else had become president.” In Washington, administration officials said they had received back-channel messages from Iran’s leadership, urging the United States to wait until the election was over for a response to Mr. Obama’s overtures. A victory by Mr. Ahmadinejad’s rival, Mir Hussein Moussavi, would have posed its own challenges, officials said, including further delays in talks until he took control of the government. And even then, it is not clear that Mr. Moussavi would be any more flexible about Iran’s nuclear ambitions than Mr. Ahmadinejad has been.  “It would be nice to have an environment without the kind of vitriol we see from Ahmadinejad,” a senior administration official said. “There clearly would be differences in tone between Ahmadinejad and Moussavi, but not necessarily in policy.” While the White House spokesman, Robert Gibbs, said the administration was monitoring “the entire situation closely,” another official said, “there’s no reason to think the regime is not in control.”

Mr. Obama, officials said, has long said he was willing to negotiate with whoever would respond to the United States, including Ayatollah Khamenei. “The administration will deal with the situation we have, not what we wish it to be,” another senior official said. For the United States, the larger problem is that while the election has frozen the dialogue, Iran’s nuclear program has speeded ahead. This month, the International Atomic Energy Agency reported that by May’s end, Iran had built and installed 7,200 centrifuges to enrich uranium and was quickly adding to its stock of nuclear fuel.  Now, the administration faces a vexing choice. It can continue to demand that Iran give up all of its enrichment capability — still the official position of the United States, but considered an all but impossible goal. Or it can tacitly accept that Iran is not going to stop enriching. “In the end, there’s an imperative to try to establish engagement,” a senior American official said. “We would certainly give them time,” he said, but added that Iran’s nuclear program would keep humming along.