Iran confirmed on March 3 that Roxana Saberi is incarcerated in Tehran's notorious Evin prison on a writ issued by the Revolutionary Court.
Authorities on March 2 said that Saberi, 31, had been working "illegally" in Iran after her press card was revoked in 2006.
Saberi has been living in Iran for the past six years and reported previously for the BBC and National Public Radio.
"We continue to request assistance from the Iranian government in identifying Ms. Saberi's whereabouts," State Department spokesman Gordon Duguid told reporters on March 3. "We are working with the Swiss Embassy in Tehran to seek additional information."
The United States does not have formal diplomatic relations with Iran.
"If Ms. Saberi is being detained by Iranian authorities, we urge the government of Iran to provide access to legal advice, a transparent judicial process, and consular access for a Swiss consular official," Duguid added.