The son of an American journalist working for Iranian state television said his mother has been imprisoned in the United States for reasons unknown.

Marzieh Hashemi, 59, an anchorwoman for Iran's English-language Press TV, was arrested on arrival at St Louis Lambert International Airport on Sunday and transferred by the FBI to a detention facility in Washington, DC, the broadcaster reported on Wednesday.

"We still have no idea what's going on," Hossein Hashemi, her elder son, told The Associated Press in a phone interview from Washington. "Everyone we ask is very vague and the information is still limited."

He said no charges have been filed against his mother, and he and his siblings were subpoenaed to appear before a grand jury.

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Hossein said his mother was shooting a Black Lives Matter documentary in St Louis, Missouri, and was about to board a flight to Denver when she was apprehended at the airport.

The FBI said in an email it had no comment on her arrest.

Marzieh was born Melanie Franklin in New Orleans and has worked for Iran's state television network for 25 years.

Hossein said his mother, an American citizen, lives in Tehran and comes back to the US usually once a year to see family, often scheduling documentary work somewhere in the country as well.

"It's important to note that she is an American citizen, she's a Muslim American citizen, she is an African American, and she has certain particular kinds of views that make it difficult for us not to think in conspiratorial sorts of ways," said Hossein Hashemi, a research fellow at the University of Colorado.

US law allows judges to order witnesses to be arrested and held if the government can prove their testimony has extraordinary value for a criminal case, and that they would be a flight risk and unlikely to respond to a subpoena.

'Apartheid and racist policy'

Marzieh had not been contacted by the FBI before she was arrested and would "absolutely" have been willing to cooperate with the agency, according to her son.

The family is trying to hire a lawyer, but it has been difficult because she has not been charged with a crime, he said.

Iran's state broadcaster held a news conference and launched a hashtag campaign for Marzieh.

"We will not spare any legal action" to help her, said Paiman Jebeli, deputy chief of Iran's state IRIB broadcaster.

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Iranian Foreign Ministry Spokesman Bahram Ghasemi told state TV that Marzieh's arrest indicates the "apartheid and racist policy" of US President Donald Trump's administration.

"We hope that the innocent person will be released without any condition," Ghasemi said.

The incident comes as Iran faces increasing criticism of its own arrests of dual citizens and other people with Western ties.

Last week, Iran confirmed it is holding US Navy veteran Michael R White at a prison, making him the first American known to be held under Trump's administration.

At least four other American citizens are being held in Iran, including Iranian-American Siamak Namazi and his 82-year-old father, Baquer, both serving 10-year sentences on espionage charges.

Iranian-American art dealer Karan Vafadari and his Iranian wife, Afarin Neyssari, received 27 and 16 years in jail, respectively, while Chinese-American graduate student Xiyue Wang was sentenced to 10 years.