US President George W. Bush strongly condemned the imprisonment of American citizens in Iran in a statement Friday and urged their release. The White House labeled the arrests as "not helpful" in resolving issues between the two countries, following historic talks earlier in the week which pointed to a diplomatic opening. The administration also on Thursday issued a travel warning cautioning US citizens, especially those holding Iranian passports, from visiting the Islamic Republic. Bush noted that four Iranian-Americans - Haleh Esfandiari, Parnaz Azima, Kian Tajbakhsh, and Ali Shakeri - are now being held against their will and a fifth American national, Robert Levinson, was missing. He criticized Iranian authorities for not providing any information on the whereabouts of Levinson despite repeated requests. Speaking of the Iranian-American dual citizens, Bush said, "These individuals have dedicated themselves to building bridges between the American and Iranian people, a goal the Iranian regime claims to support. Their presence in Iran -- to visit their parents or to conduct humanitarian work -- poses no threat." Bush's statement comes at the end of a week which began with America sitting down with Iran for its first formal high-level talks in close to three decades. Despite the historic event, the Iranians then charged the detainees with crimes including espionage. "It is not helpful to resolving these outstanding issues we have with Iran, whether it is Iran activity in Iraq that destabilizes that nation, or progress on the nuclear issue, for Iran to be capturing innocent Americans," US National Security Advisor Steve Hadley said at a press briefing Friday. "It underscores the character of this regime, and it underscores the problem we have for those people who say, well, 'Why don't you talk to Iran?' It is a good reminder at how difficult this regime is, and of the kinds of policies its pursuing."