Iran's disputed presidential election poses additional challenges to the Obama administration, which has stressed diplomacy as its preferred means to deal with Tehran's nuclear ambitions.
With tens of thousands of Iranians taking to the streets in protest, Vice President Joseph R. Biden Jr. expressed doubts that Mahmoud Ahmadinejad was legitimately re-elected to the presidency in Friday's vote.
"There's an awful lot of questions about the way this election was done," he said Sunday.
But Mr. Biden also indicated that the White House was willing to pursue nuclear negotiations with Iran, regardless of how the election dispute plays out.
"Look, the decision has been made to talk," he said on NBC's "Meet the Press."
The White House declined to elaborate on how it will proceed regarding President Obama's engagement strategy with Iran.
Official results had Mr. Ahmadinejad winning more than 60 percent of the votes cast and even carrying the home district of his main rival, Mir Hossein Mousavi, a former prime minister and foreign minister who campaigned with his wife and was widely seen as an advocate for reform and better relations with the West.
Mr. Mousavi has challenged the official results and said that he was told Friday night that he had won by a substantial margin. He has made a formal request to the nation's religious authorities to annul the results and urged supporters to express themselves peacefully.
Although Iran's top leader -- Ayatollah Ali Khamenei -- is said to be in charge of Iran's foreign and defense policy, an Ahmadinejad win could make it even harder for the Obama administration to engage Iran in negotiations.
Under Mr. Ahmadinejad's watch, Iran has accelerated its nuclear program, and the Iranian president has denied or minimized the Holocaust and said that Israel should be "wiped from the pages of history" as a Jewish state.
Kenneth Pollack, acting director of the Saban Center at the Brookings Institution, said, "It could be that in the case of Iran, only Nixon can go to China and only Ahmadinejad and Iran's hard-liners would have the political standing to strike a deal with the United States. But this still begs the question of whether the hard-liners want a deal with the United States."
Mr. Mousavi -- who was prime minister in the 1980s when Iran revived a nuclear program that had begun, with U.S. assistance, under Iran's deposed monarchy -- has said he would pursue nuclear negotiations with the United States but would seek to retain a uranium enrichment program.
In contrast to Mr. Ahmadinejad, however, Mr. Mousavi has pledged a less confrontational foreign policy that would reduce Iran's international isolation.
The protests, which continued Sunday, are the largest since July, 1999, when university students protested the closure of a reform newspaper and the killings of several students by security forces in Tehran.
"It's possible that we are seeing the beginning of a second Iranian revolution, but revolutions are impossible to predict, and if history is any guide, the regime will crack down with as much force as necessary, and the Iranian people will back down," Mr. Pollack said.
A U.S. intelligence officer, who asked not to be named because he was disclosing sensitive information, said that the U.S. has confirmed that Akbar Hashemi Rafsanjani, a former Iranian president and key backer of Mr. Mousavi, met with Ayatollah Khamenei on Friday and was told that security forces would crack down harshly on continued protests.
Sen. Joe Lieberman, Connecticut independent, urged the Obama administration to speak "loudly and clearly" for the Iranian people, calling the election a "mockery of democracy."
"We as Americans have a responsibility to stand in solidarity with people when they are denied their rights by repressive regimes," he said. "When elections are stolen, our government should protest. When peaceful demonstrators are beaten and silenced, we have a duty to raise our voices on their behalf."
But Suzanne Maloney, an Iran specialist at the Saban Center, said that the one lesson the U.S. has learned from the democracy-promotion efforts of the Bush administration -- in which she served -- is that such movements have to be homegrown.
"These guys are going to fight their own battles," Ms. Maloney said.
The Bush administration announced a program to support democracy on the ground in Iran in 2006, but leading Iranian opposition figures such as Akbar Ganji said at the time that the money would mark them as targets for the Iranian authorities.
The election has attracted intense interest from Iranians abroad who are eligible to vote in Iran if they also hold Iranian passports. About 100 Iranian-Americans gathered Sunday outside an office in upper Georgetown that provides passports and other services to Iranian expatriates. Like demonstrators in Tehran, the protesters held placards that read: "What about my vote?"
Karim Sadjadpour, of the Carnegie Endowment for International Peace, said the Obama administration is wise to wait to react.
"Once the dust settles, the United States will eventually have no choice but to talk to Tehran, but it will likely be a cold, hard-nosed dialogue rather than friendly greetings."
He even suggested that U.S. officials should announce publicly that they wish to talk directly with Ayatollah Khamenei.
• Barbara Slavin contributed to this report.