Exiled Iranian dissidents, human rights campaigners and Iranian-American advocacy groups are fearing for their lives as they receive what they say are ongoing death threats from Iranian intelligence agents and regime sympathizers working in the United States.

"I have to change my phone number every month because Iranian intelligence are threatening to kill me," human rights activist Ahmad Batebi, who fled to the U.S. from Iran in 2008, told FoxNews.com. Now working with the Voice of America, the international broadcasting service funded by the U.S. government, Batebi says he spent nine years in Iranian prisons, where he was tortured for his affiliation with student groups working against the Tehran government.

He said the Iranian agents who are threatening him are working under diplomatic cover at the Iranian mission to the United Nations or coming here on political visas to spy on regime opponents. The U.S. broke formal diplomatic relations with Iran in 1980.

"They say, 'You are not safe in the U.S. or anywhere, and if we don’t get you in America we will get you in Europe,'" he said.

Another Iranian, Mohsen Sazegara, a former politician-turned-activist who now lives in Virginia, says he routinely receives death threats and was singled out recently by Iran’s deputy minister for intelligence, who Sazegara says declared, "We will catch you wherever you are."

He said silencing human rights advocates is Tehran's goal, and he accused Iran's "cyber army" of hacking his Virginia-based Web site and his YouTube account, on which he posted videos critical of the regime. (YouTube did not return calls seeking comment.)

The threats have not been limited to activists, others say.

“I’m worried about the safety of my children,” said a prominent Iranian-American democracy advocate in Washington, who asked to remain anonymous because he says he and his family routinely receive death threats from male callers speaking Farsi.

“Iran keeps a very close eye on the Iranian Diaspora in the United States, and I’m concerned that they will act on these threats,” he said.

He said he has contacted the FBI and turned over voice mails left on his answering machine to police.

Mary Ann Jennings, a spokeswoman for the Fairfax County Police Department in Virginia, said the man had turned over MP3 audio files containing the threats, and "The case has been turned over to our criminal investigators."

An FBI spokeswoman said the bureau would not comment on whether it was investigating the threats.

But former CIA agent Robert Baer, who maintains contact with Iranian opposition groups, said Iranians living in America have reason to be afraid.

He said Iran has a history of murdering its opponents in Europe, and it has maintained informal networks of illegal operatives in the U.S. for years.

“If the Iranians think that somebody is a threat to the regime, they will kill them here. These are guys you don’t want to mess with,” said Baer, a 21-year CIA veteran with extensive experience in the Middle East.

“As the Iranian regime becomes more desperate it is more willing to take chances. They have people who can reach out and touch people here in the States,” he said.

John M. Cole, a former FBI counter-intelligence agent, said Tehran's agents have been targeting dissidents in the U.S. for years. And he said he is concerned that Iranian agents could move beyond threats as the volatility in their country worsens.

Though there is no record of Iranian dissidents being assassinated in the U.S. so far, Cole said, "We've had investigations and incidents in the past where Iranians were recruiting émigrés to put pressure on people here in the U.S. If the Iranians find people who are spewing anti-government sentiment, they look for ways to quiet them."

Baer said it’s difficult for the FBI to investigate the cases. Unless the bureau has a crime scene or a body, he said, the investigations usually go nowhere.

“The FBI doesn’t know what to do with it. It’s very hard for them to figure out dissident politics,” he said.

Hooshang Amirahmadi, director of the American-Iranian Council in Princeton, N.J., a non-profit group promoting relations between Iran and the U.S., said, "We know the regime has agents here in the U.S. who are usually working with official Iranian institutions like the United Nations, and there are also infiltrators.

"It's a very sensitive issue. They’re trying to discourage the dissidents from speaking out, and many of them are afraid for their lives."

Tehran's campaign has spread fear among Iranians in the U.S., many of whom refuse to go public because of threats and arrests of family members and friends still in Iran, Amirahmadi said.

It's even worse on in Europe, said Iranian human rights activist Ladan Boroumand, whose father Abdorrahman Boroumand, a prominent critic of Tehran, was murdered in 1991 in Paris. Iranian dissidents and French prosecutors said Iranian agents were behind the hit.

Boroumand, who splits her time between Washington and Paris, said Iran’s interior ministry maintains a “black list” of groups and people it accuses of waging war against the regime, declaring them enemies of the Iranian state.

A U.S. State Department official told FoxNews.com, "I think it's fair to say that we are concerned about threats made to Iranian dissidents in the U.S., and feel that all such threats should be referred to appropriate law enforcement officials."

Batebi says he has received little help from the police. He says they told him that unless he recorded the threats, there wasn’t much they could do.

He and others need to watch their backs, Iran watchers warn, saying Tehran will mark dissidents for death no matter where they live.

"I would be worried," Baer said. "It is more than conceivable their lives could be in danger."