Liz Truss has demanded the “immediate release” of Nazanin Zaghari-Ratcliffe, ahead of meeting her Iranian counterpart on Wednesday.
The newly-appointed Foreign Secretary has vowed to raise the case of the 42-year-old British-Iranian mother of one, who is trapped in Iran, when she holds talks with Iranian foreign minister Hossein Amir-Abdollahian.
The two ministers are set to meet at the United Nations General Assembly in New York after talks were postponed earlier this week.
Talking to reporters on the train to Washington DC on Tuesday, Ms Truss said she would be “hard headed” and “proactive” in her approach to securing the release of detained British-Iranians.
“I've already spoken to the families of the detainees in Iran. It's completely unacceptable that they're being held there and I'm going to be very proactive in pushing for their release,” she said.
Asked what she will say to the Iranians, she said: “I’m pushing for her immediate release.”
Ms Truss is also expected to bring up the cases of Anoosheh Ashoori and Morad Tahbaz, and will call on Tehran to comply with its nuclear commitments.
Ms Zaghari-Ratcliffe's husband Richard reported he had a “good” discussion with Ms Truss during a 10-minute phone call at the weekend, as he urged the Foreign Secretary to make his wife’s case her top priority.
The British dual national has been in custody in Iran since 2016 after being accused of plotting to overthrow the government.
Mr Ratcliffe believes his wife is being used as a pawn in Tehran’s geopolitical manoeuvres and has described Tehran’s conduct as “hostage-taking”.
Sacha Deshmukh, chief executive of Amnesty International UK, said earlier this week that Ms Truss must set out a “clearly articulated strategy for securing the release of British nationals arbitrarily detained in Iran”.
He added that the plight of Ms Zaghari-Ratcliffe and other British nationals had “extended across the last four foreign secretaries”, as he warned: “It’s long past time that the UK finally brought this deeply distressing episode to an end.”