US President George W Bush has condemned the detention of four American citizens in Iran and called for their immediate release.
He said they had gone to Iran "to visit their parents or to conduct humanitarian work" and posed no threat.
Academic Haleh Esfandiari, social scientist Kian Tajbakhsh and journalist Parnaz Azima are accused of spying.
A fourth Iranian-American, businessman Ali Shakeri, is believed to have been arrested but not officially charged.
"They should be freed immediately and unconditionally," President Bush said.
He also demanded to know the whereabouts of former FBI official Robert Levinson, who went missing in March while visiting Iran.
The president added: "The United States is committed to protecting its citizens at home and abroad."
The three who have been charged with espionage could face the death penalty if found guilty.
Tehran has accused Washington of using intellectuals and others to try to bring about what it calls a velvet revolution aimed at undermining the Islamic authorities.
The US denies the accusation.
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