WASHINGTON (Reuters) - The U.S. State Department said on Wednesday it is looking into reports that Iran has detained a third U.S. citizen of Iranian descent but declined to provide details.

Iran on Sunday confirmed the detention of an unnamed visiting U.S. citizen, the latest in a string of arrests of dual nationals, but declined to say what charges they might face, the semi-official Fars news agency reported.

However, an Iranian Judiciary spokesman said the person's arrest took place in the northeastern city of Gorgan and that the trial "may be held" in Tehran, Fars reported.

An Iranian-American man, California-based Robin Reza Shahini was detained while visiting his mother in Gorgan earlier in July, according to Shahini's friends.

"We've seen reports of the detention (in) Iran of a person reported to be a U.S. citizen, and I can tell you we are looking into that," State Department spokesman John Kirby told reporters, saying he had no more information to offer.

The two other U.S. citizens of Iranian descent detained in Iran are businessman Siamak Namazi and his father Baquer Namazi. Iran does not recognize dual nationality, which prevents Western embassies from visiting such detainees.

Iranian dual nationals from the United States, Britain, Canada and France are behind bars on various charges, including espionage or collaborating with a hostile government. Some are kept to be used for a prisoner exchange with Western countries, according to prisoners, their families and diplomats.

(Reporting by Arshad Mohammed; Editing by Sandra Maler)