"The city remains the most likely venue for global tensions with Iran to spill over onto American soil," Silber told the House Homeland Security Committee.
Tensions with Iran have increased over the country's unwillingness to scale back its nuclear program.
Last month, Homeland Security Secretary Janet Napolitano said that she was concerned that Hezbollah would attempt a terrorist attack on American soil and that she had been in touch with U.S. Jewish groups. Napolitano said she wasn't aware of any specific threats to the groups or other U.S. targets.
Government officials estimate "hundreds" of Iranian and Hezbollah operatives are in the United States, said Rep. Peter King (R., N.Y.), the homeland security panel's chairman.
"We have a duty to prepare for the worst," he said.
Rep. Bennie Thompson of Mississippi, the panel's senior Democrat, complained that Obama administration officials weren't among the witnesses at the hearing.
"I am concerned about whether the testimony we received will be based on current information," said Thompson. "We should not engage in a public discussion that creates fear and delivers misinformation."
New York police are facing criticism for conducting surveillance of Muslim communities. Human Rights Watch on Tuesday requested in a letter to Attorney General Eric Holder that the Justice Department formally investigate the city department's actions.
The United States and Europe have tightened economic sanctions on Iran since a Nov. 8 U.N. atomic inspectors' report raised questions about Iran's nuclear program.
The Iranian surveillance has gone on for years, Silber said. In early 2010, federal air marshals found four people who said they worked for the Islamic Republic of Iran Broadcasting Co. videotaping and photographing the Wall Street heliport, he said. One person held a camera at waist level, focusing on the structure and not the helicopters in the air, he said.
Several members of the Iranian delegation to the United Nations in 2008 were seen taking pictures of the Metropolitan Transportation Authority train tracks inside Grand Central Station, Silber said. In the early to mid-2000s, police interviewed people with ties to the Iranian government who were taking pictures and videotaping infrastructure, he said.
Hezbollah's presence in the New York region has been uncovered in investigations, Silber said. Twenty-six people, including a former Brooklyn resident, were indicted in 2009 for conspiring to provide material support to Hezbollah by obtaining weapons, and raising money through the sale of fraudulent passports and other schemes, Silber said.