The US has rejected Iran's suggestion of a prisoner swap for several Iranians jailed in the US for the three American hikers currently being held in Iran.
US Secretary of State Hillary Clinton said the hikers should be released immediately on humanitarian grounds.
Her remarks came a day after Iran's President Mahmoud Ahmadinejad suggested that talks were under way regarding such a swap.
Mrs Clinton called Iran's allegations against the hikers "unjust".
Iranian officials have previously alleged that the three were spying.
"There are no negotiations taking place between the United States and Iran. We believe they should unilaterally release our detained citizens," Mrs Clinton said at the State Department after a meeting with Bahrain's foreign minister.
The three hikers were jailed in Iran in July for allegedly straying across the country's border while hiking in Iraq.
The case of the hikers is not comparable to those of Iranians PJ Crowley told Reuters news agency earlier.
"There is not really an equivalence, if you will, between, say an Iranian citizen who has been indicted and/or convicted of arms trafficking in violation of international law and three hikers who wandered across an unmarked border," he said.>>>
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