Iranian state media are confirming Tuesday that Iran has detained three Americans on charges of entering from Iraq without permission.
Iranian officials did not tell news agencies the identities of the Americans.
U.S. Secretary of State Hillary Clinton has urged Iran to provide information about three missing tourists, who are believed to have been detained Friday in Iran.
Clinton told reporters Monday that the U.S. is working through the Swiss embassy in Tehran to determine the trio's whereabouts and bring them home. The United States and Iran do not have diplomatic ties.
The head of security in northern Iraq's Kurdish region Hakem Qadir says the three went to see a waterfall near the Iraq-Iran border, but apparently ventured into neighboring Iran.
They were surrounded by Iranian soldiers and arrested.
The border in the mountainous region is not well-marked.
Kurdish officials say at least two of the detained hikers had been studying Arabic in Syria. They had entered Iraq's semi-autonomous Kurdish region from Turkey.
Some information for this report was provided by AFP and Reuters.