The US secretary of state has appealed to Iran for information on three Americans arrested after apparently straying across the Iraqi border.
Hillary Clinton said she was “concerned” and called on the Iranian authorities to find the trio.
Iranian officials have accused the three of ignoring warnings from border guards and crossing into Iran from Iraq’s Kurdish region on Friday.
The frontier between Iran and Iraq in the region is said to be poorly marked.
“We call on the Iranian government to help us determine the whereabouts of the three missing Americans and return them as soon as possible,” Mrs Clinton said.
She said the US still did not have official confirmation that the Iranian government was holding them.
She said Swiss officials, who look after US interests in Tehran, had not yet received confirmation of the trio’s arrest, despite requests.
On Monday, two of the Americans were named by relatives – Shane Bauer, a Middle East-based freelance journalist from Minnesota, and Joshua Fattal from Pennsylvania, whose father is Iraqi.
The identity of the third American has not yet been confirmed, although Iraqi officials and US media have named her as Sara Short.
“Our family is concerned about the safety and welfare” of the three, Mr Bauer’s mother, Cindy Hickey, told the Associated Press.
A Kurdish government official has told the BBC the Americans entered the region as tourists on a walking tour on Tuesday and spent two nights at a hotel in Suleymaniyeh.
They then travelled to the resort of Ahmed Awa, where they apparently ignored local warnings not to climb a mountain close to the border on Friday.
A fourth member of the travelling party, Shon Meckfessel, did not join the hike because he was ill.
The BBC’s Jon Donnison in Washington says the detentions are possibly the last thing the US government wants given how strained relations already are by the dispute over Iran’s nuclear ambitions and its recent presidential election.