The government says there is no specific or credible threat, but law enforcement officials from Los Angeles to New York City are keeping an eye out for potential Iranian operatives.
Los Angeles, which has one of the largest Iranian communities outside Iran, has moved potential Iranian threats to the top of its intelligence briefings over the past few weeks. The New York Police Department said it assumes Iran would attack the city, with its especially large Jewish population.
Intelligence experts say Jewish organizations, in particular, could be targeted.
Iran has accused Israel of killing some of its nuclear scientists, while Israel has warned of a military strike against Iran's nuclear energy program over concerns it could lead to development of a nuclear bomb.
Amid the tensions, Los Angeles increased its outreach to Iranian and Jewish communities, assuring them there is no reason to be paranoid or overly anxious, said Deputy Chief Michael Downing, commander of the Los Angeles Police Department's counterterrorism and special operations bureau.
The chairman of the House Homeland Security Committee, Rep. Peter King, R-N.Y., said Hezbollah and Iran are capable of attacking inside the U.S. King said he received two intelligence briefs on the Iranian threat in the past two weeks.
Iran has been blamed for violent acts in the U.S. before. In 1980, for instance, an American convert to Islam gunned down an Iranian critic of Iran's religious leaders in Bethesda, Md.
The Associated Press contributed to this report.