The three are thought to have strayed over the border from Iraq
Three Americans arrested after apparently straying into Iran are being questioned by police, Iranian state television has reported.
"We don't know whether they are tourists or not. We are questioning them," said a local security official.
Iranian officials have accused the three of ignoring warnings from border guards and crossing over from Iraq's Kurdish region on Friday.
The frontier between Iran and Iraq in the region is said to be poorly marked.
Iraj Hassanzadeh, the deputy governor of Iran's Kurdistan province, told Iran's Fars news agency the trio were arrested on the Malakh-Khor border, near the town of Marivan, but that their identities were not yet known.
"Anyone who seeks to illegally enter the country from the Kurdistan border will be arrested," he said.
The detained Americans have been named by relatives as Shane Bauer, a Middle East-based freelance journalist from Minnesota, his girlfriend, Sarah Shourd, and Joshua Fattal from Pennsylvania, whose father is Iraqi.
All three are reported to be experienced travellers
Mr Bauer, an Arabic speaker, had been studying the Middle East and planned to report on forthcoming Kurdish elections in Iraq for the California-based New America Media (NAM), said the group's executive director, Sandy Close.
Ms Close said Mr Bauer was a "gifted young correspondent" and had not had any plans to cross into Iran.
"How he'd gone into the hills, how he wound up crossing the border I have no idea, but never in the year we've been in correspondence has he indicated any interest in covering Iran," she told the Associated Press, adding that Mr Bauer did not speak Farsi.
"It's unthinkable to me that he would have just simply decided on some crash adventure," she said.
Mr Fattal's mother, Laura, told CNN she and her husband were "eager for the best welfare and conditions" for their son and his companions.
On Monday, US Secretary of State Hillary Clinton said she was "concerned" and appealed for their safe return.
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.