The US has rejected Iran's offer to swap several Iranian prisoners jailed in the US for the three American hikers currently being held in Iran.

US secretary of state Hillary Clinton told reporters on Wednesday that a prisoner transfer was out of the question and that the hikers should be released on "humanitarian" grounds as their continued detention was "baseless".

Mrs Clinton set out an uncompromising US stance at a press conference after a meeting with Bahrain's foreign minister Sheikh Khalid bin Ahmed al-Khalifa.

She told the gathered reporters: "There are no negotiations taking place between the United States and Iran. We believe they should unilaterally release our detained citizens."

Her remarks came day after Iranian president Mahmoud Ahmadinejad suggested in a television interview that talks between the countries over a possible prisoner swap were ongoing.

US State department spokesman Philip Crowley had previously denied these claims, saying on Tuesday: "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."

The three Americans were arrested in late July 2009 after they crossed Iran's northern border with Iraq's autonomous Kurdish region. Iranian authorities have accused them of spying and said they would be tried but Tehran is yet to announce a date for their trial.

The US citizens, Joshua Fattal, Shane Bauer, and Sarah Shourd, maintain their innocence and said they were on a hiking trip in Iraq and accidentally strayed into Iranian territory. They have had no contact with their families since their arrest.