Iran has arrested three Americans who "infiltrated" the border with Iraq, state-owned Al-Alam television said on Saturday, in the Islamic republic's first confirmation of the incident.
"An informed Iranian source confirmed the arrest of three Americans after they infiltrated through the Iraqi border," the Arabic-language television station reported.
In northern Iraq, a Kurdish official said earlier that three US backpackers were arrested after having been warned on the Iraqi side not to hike in the mountains because of the proximity of the border with Iran.
Beshro Ahmed, media adviser for general security in Iraq's autonomous Kurdish region, said the two men and a woman had entered from Turkey earlier along with a fourth American who did not join the trek because he was ill.
According to CNN, Iran detained the three US nationals on Friday after they crossed into the country from Iraqi Kurdistan.
Ahmed named the three as Shane Bower, Sara Short and Joshua Steel, while Shaun Gabriel Maxwell stayed behind in their hotel in the Kurdish region's second largest city of Sulaimaniyah.
"On Thursday, three of them went to the summer resort at Ahmed Awa," Ahmed said of an area about 90 kilometres (55 miles) northeast of Sulaimaniyah.
Ahmed Awa is a cool and heavily-forested area popular with Iraqis seeking to escape the country's high summer temperatures.
The mountainous region has several youth hostels, one of which the three American tourists stayed at, but the nearby border with Iran is not clearly marked.
"The (Kurdish) tourist police in the area asked them not to climb the mountains because the Iranian border was very close," Ahmed said.
"On Friday, they went close to the mountains, and climbed them. Then they called their friend in the hotel telling him that they were arrested by Iranian forces at the border," Ahmed said.
"Shaun was in the hotel and he called the US embassy in Iraq to tell them about this information, and the Americans came to the hotel and took him."
Speaking to AFP earlier, a US embassy spokeswoman in Baghdad said: "We've seen the reports and are looking into it but can't confirm anything at this time."
Iranian state television, without quoting a source, said on an earlier Saturday news bulletin that three US military personnel had gone missing near the border.
But an official at the Pentagon in Washington has insisted that no US military personnel were involved.
US-Iran tensions have been high for years, with the Islamic republic deriding the United States as the "Great Satan" and President Barack Obama's predecessor, George W. Bush, labelling Tehran part of an "axis of evil."
The acrimony has increased as Washington, despite Obama offering Tehran a chance to turn the page, has expressed mounting concern over Iranian nuclear ambitions and its disputed June presidential election.