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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.