Austin Tice, 31, crossed into Syria in May through the Turkish border and has not been heard from since mid-August, The New York Times reported.
Eva Filipi, the Czech ambassador to Syria, told a Czech television station Monday sources report "he is alive and ... was detained by government forces on the outskirts of Damascus where the rebels were fighting government troops."
The Czech embassy in Damascus has been acting on behalf of U.S. interests in Syria since the U.S. embassy was closed in February.
Others familiar with Tice's whereabouts corroborated Filipi's account.
The Syrian government has not responded to official requests for information about Tice, the U.S. State Department said. The department said it would continue to work through the Czech embassy to learn more about Tice's situation.
Two journalists with al-Hurra TV are also missing. They were last seen Aug. 20 in Aleppo.
Ten journalists, including five foreigners, have been killed in Syria since the conflict began in March 2011, Reporters Without Borders said. At least 30 Syrian citizen journalists also also have died.