The mountains were once a popular destination for Iraqi tourists
The US says it is using all available means to assist three Americans arrested inside Iran after apparently straying across the Iraqi border.
Iran accuses the three of ignoring warnings from border guards and crossing into its territory on Friday from Iraq's Kurdish region.
Washington has asked the Swiss, who look after US interests in Teheran, to seek consular access to the three.
The border between Iran and Iraq is reportedly poorly marked.
A Kurdish government official, Bayan Abdurahman, told the BBC the Americans had entered the region as tourists on a walking tour.
They had travelled to the resort of Ahmed Awa, where apparently they ignored local warnings not to climb a mountain close to the border.
The BBC's Jon Donnison, in Washington, says the detentions are possibly the last thing the US government wanted given how strained relations already are by the dispute over Iran's nuclear ambitions and its recent elections.
Earlier, a senior security official in the Iraqi city of Suleymaniyeh, Qadr Hamajan, told Reuters news agency that the three US nationals and a fourth man had arrived there on Wednesday.
He said three of them had travelled the following day to the tourist resort of Ahmad Awa near the Iranian border. They went out hiking near Halabja.
The fourth was taken ill and stayed behind in a hotel in Suleymaniyeh.
"They phoned their friend ... and told him 'we are in trouble and are being held by soldiers who are not speaking Kurdish or Arabic'," Mr Hamajan said.
An official in the region told the BBC that the Iraqi border guards had seen three Americans with big backpacks crossing the border, but they did not stop them because they thought Americans were allowed to go anywhere.
The mountainous border region is popular for hiking, and the American walkers had no interpreter or bodyguards.
Reports earlier said American helicopters and vehicles searched the area amid fears the three had crossed the border.