An American journalist working for Iran's state television channel Press TV, who was arrested in the United States, will appear in a court on Friday.

Iran has called for the immediate release of TV anchor and documentary filmmaker Marzieh Hashemi, 59. Her employer, the English-language channel Press TV, said she was arrested on Sunday by the FBI at St Louis Lambert International Airport.

The FBI has not commented on the case.

"Marzieh Hashemi, detained by the FBI on unspecified charges, is due to appear in a Washington, DC, court on Friday," Press TV reported on Friday, without elaborating.

Hashemi was taken into custody in St Louis, Missouri, where she had filmed a Black Lives Matter documentary before being transferred to a prison in Washington, it said.

'Spurious grounds'

Suzanne Nossel, from the group PEN America that promotes free expression in writing, said she is concerned Hashemi might have been targeted for her work on the Black Lives Matter movement, or for political reasons.

"It would be shocking to see the US following the model - often employed by China and Iran - of detaining citizens of a country they consider hostile on spurious grounds, including to serve as a bargaining chip," Nossel said in a statement.

Hashemi was quoted as saying by the broadcaster she was prevented from observing her Islamic faith and wearing her headscarf. She was also reportedly given food that was not halal.

Press TV on Wednesday quoted Hashemi's son as saying the journalist, who had been living in Iran for more than a decade, was arrested as a "material witness" to a criminal case and no formal charges had been made against her.

US federal law allows the government to arrest and detain a witness if it can prove their testimony is material to a criminal proceeding and it cannot guarantee their presence through a subpoena.

According to Press TV, Hashemi was born Melanie Franklin in the US and changed her name after converting to Islam. She travelled to the US to visit her family, the channel said.

'Duty to defend'

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

"She is the wife of an Iranian citizen and we see it as our duty to defend the rights of our citizens," Zarif told Iran's Arabic-language state broadcaster Al-Alam news channel.

In a statement on Friday, the New York-based Committee to Protect Journalists (CPJ) called on the Justice Department to disclose the reason for Hashemi's arrest.

CPJ also noted: "Iran routinely jails journalists, with at least eight behind bars in relation to their work."

Tensions have been high between Iran and the US since President Donald Trump's decision to pull out of an international nuclear deal and re-impose sanctions on Tehran.

Several Iranian dual nationals from the US, the United Kingdom, Austria, Canada and France have been arrested in the past years in Iran and are being kept behind bars on charges including espionage and collaborating with hostile governments.