Iran's powerful intelligence ministry has stepped up its war of nerves with the west by telling the country's academics they will be suspected of spying if they maintain contact with foreign institutions or travel abroad to international conferences.

The blunt warning has been issued by the ministry's counter-espionage director in an atmosphere of rising suspicion and paranoia as Iran claims to have cracked a CIA-backed spy ring and has charged three American citizens with spying.

In a briefing with Iranian journalists, the official - whose identity was not disclosed - accused western intelligence agencies of using academic contacts to lure scholars into an espionage network against Iran. He said seminars inside and outside the country were used.