Three missing US nationals have been detained by Iranian guards after they crossed into Iran from Iraq, Iranian media confirmed.
The two men and a woman, described as tourists by Iraqi media, were reported to have been hiking in Iraq's Kurdish region which borders Iran.
Iranian media said the three were arrested inside Iran.
The Iraq-Iran border is generally well guarded but is not well marked in this area, correspondents say.
Iran's state-owned Al-Alam confirmed the arrests, quoting an informed source. No other details were given.
The US embassy in Baghdad said it was investigating and the US state department earlier said: "We are using all available means to determine the facts in this case."
But the BBC's Jon Donnison, in Washington, says news of the detentions will be met with considerable dismay and is 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.
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 arrived there on Wednesday.
He said three of them travelled the following day to a the tourist resort of Ahmad Awa near the Iranian border. They went out hiking near Halabja.
The fourth had been 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 saw 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.