US Secretary of State Hillary Clinton has called on Tehran to help determine the whereabouts of three Americans believed to have been arrested by Iranian authorities last week.
The three are thought to have strayed over the border into Iran while hiking in the Kurdish region in northern Iraq on Friday.
Mrs Clinton says they should be returned as quickly as possible.
She says Swiss diplomats, who represent American interests in Iran, have not yet received confirmation of the arrests.
Iran accuses the three of ignoring warnings from border guards and crossing into its territory from Iraq.