The U.S. government has expelled two Iranian security guards at Iran's U.N. Mission, citing activities "incompatible with their stated duties," a U.S. official said Tuesday.

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 was in November 2003; after they were stopped recently while taking photographs, the government asked them to leave the country, the official said.

The pair left the United States in the last few days, the official said.

Richard Grenell, spokesman for the U.S. Mission to the United Nations (search), 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.

Iran is one of seven nations on the State Department list of countries that are state supporters of terrorism (search).

The United States accuses Iran of trying to build nuclear weapons, and President Bush has labeled Iran part of an "axis of evil" with North Korea and prewar Iraq.

Secretary of State Colin Powell hinted last week that Iran faced the prospect of U.N. economic sanctions if it did not prove to the world it has no nuclear weapons.

Earlier this month, the U.N. nuclear agency rebuked Iran for covering up its programs and warned it had little time left to disprove it had a nuclear weapons program.

The United States broke off ties with Iran in 1979 after militant students seized the U.S. Embassy in Tehran and held its staff hostage for 444 days to protest Washington's refusal to hand over the shah for trial.

Hard-liners and reformers have long been split over whether to resume full ties with the United States.