The warning comes after international human rights groups called on Iran to release two Iranian-American scholars -- Haleh Esfandiari and Kian Tajbakhsh -- and peace activist Ali Shakeri. All three are being held on suspicions of spying or acting against state interests.
A fourth, Parnaz Azima, a reporter with Radio Farda, a joint project of Voice of America and RFE/RL -- has been charged with disseminating propaganda and is free on bail awaiting trial.
Amnesty International, Human Rights Watch, and Nobel Peace Prize laureate Shirin Ebadi, among others, warn that Iranian authorities are trying to sow fear among journalists, writers, scholars, and activists.
The United States has repeatedly denied Iran's accusations that the four are spies or employees of the U.S. government.
U.S. State Department spokesman Tom Casey said Iran has not responded to requests to let Swiss diplomats visit with the prisoners. Switzerland represents U.S. interests in Tehran.
(AP, Radio Farda)