LONDON, Jan 22 (Reuters) -

Iran summoned the Swiss ambassador to protest the arrest in the United States of a journalist who worked for an Iranian English-language television channel, an Iranian news agency reported on Tuesday.

Iran asked the envoy for the immediate release of Marziyeh Hashemi, an American-born employee of Press TV, the Fars news agency reported, quoting Iranian foreign ministry spokesman Bahram Qasemi.

The Swiss embassy represents U.S. interests in Tehran, where Washington has had no mission since 1980.

Hashemi, 59, was arrested on Jan. 13 by the Federal Bureau of Investigation at St. Louis Lambert International Airport. Her son said she was detained as a "material witness" in a criminal case and no formal charges had been brought against her.

Federal law allows the U.S. government to detain witnesses if it can prove their testimony is material to a criminal case and that it cannot guarantee their presence through a subpoena.

The FBI has not commented on the case.

"In the meeting (with the Swiss ambassador), we expressed our concerns about the FBI's treatment of Mrs Hashemi, and the questionable silence of the American officials about this case, which are against U.S. international commitments and a breach of human rights," Qasemi said.

According to Press TV, Hashemi was born Melanie Franklin in the United States and changed her name after converting to Islam. She had been living in Iran for more than a decade and travelled to the United States to visit her family.

Press TV said Hashemi will appear before a grand jury on Wednesday.

Iran's Foreign Minister Mohammad Javad Zarif condemned the arrest a "violation of freedom of speech and unacceptable".