The language is reserved for cases involving espionage.
The Iranians were caught on three occasions taking photos of infrastructure, transport systems and New York City landmarks, the official said, speaking on condition of anonymity.
The first incident was in June 2002 and the second in November 2003, the official said. After they were stopped recently while taking photographs, the government asked them to leave the country, the official added.
The two Iranians left the United States in the last few days, the official said.
Richard Grenell, spokesman for the U.S. Mission to the United Nations, confirmed the expulsions but gave no details.
"We asked them to leave because we were very concerned about their activities, which were incompatible with their stated duties," he said.
By Edith M. Lederer