President Bush on Friday demanded that Iran "immediately and unconditionally" release four Iranian-Americans detained for alleged espionage and provide information about a former FBI agent missing in the country.

"I strongly condemn their detention at the hands of Iranian authorities," the president said in a written statement, stepping up pressure on Tehran over the cases.

The United States has denied that the four detainees are spies or employees of the U.S. government. The State Department on Thursday warned U.S. citizens against traveling to Iran, accusing its Islamic authorities of a "disturbing pattern" of harassment of Iranian-Americans.

The four detained scholars and activists are Haleh Esfandiari, director of the Middle East Program at the Washington-based Woodrow Wilson Center for Scholars; Kian Tajbakhsh with George Soros' Open Society Institute; journalist Parnaz Azima from the U.S.-funded Radio Farda; and Ali Shakeri, a peace activist and founding board member at the University of California, Irvine, Center for Citizen Peacebuilding.

"These individuals have dedicated themselves to building bridges between the American and Iranian people, a goal the Iranian regime claims to support," Bush said. "Their presence in Iran — to visit their parents or to conduct humanitarian work — poses no threat."

At the State Department, deputy spokesman Tom Casey said Tehran has still not responded to repeated requests for access to the detainees by Swiss officials who act as intermediaries for the United States in Iran because the two nations do not have diplomatic relations.

"This is hardly the stuff of espionage, this is hardly the stuff of government disputes," he told reporters.

"It is absolutely incredible to us to think that there could be any possible doubt in the Iranians' minds that these individuals are there simply to conduct normal, basic human interactions, including family visits," Casey said.

Bush's statement also said he was "disturbed" by the fact that Iran has still not provided any information about the welfare and whereabouts of former FBI agent Robert Levinson who went missing in Iran while on private business there in March.

"I call on Iran's leaders to tell us what they know about his whereabouts," the president said.