NAZANIN Zaghari-Ratcliffe has asked why she is still incarcerated in Iran more three years after being detained - after Australia secured the release of two citizens from the country after just three months.

Mrs Zaghari-Ratcliffe, who was arrested in Iran in 2016 and given a five-year sentence for allegedly plotting against the Iranian government, has raised the issue following the release of Jolie King, a dual British-Australian citizen, and her Australian boyfriend Mark Firkin last week, some three months after being arrested.

The British-Iranian mother, whose sister-in-law Rebecca Ratcliffe is a GP in Cwmbran, had come to know Ms King after the two were housed in the same area of Tehran's notorious Evin prison.

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Ms King and Mr Firkin had been blogging and vlogging on their extended journey across Asia, were arrested on spying charges after flying a drone near a military base, Iranian officials confirmed.

Their release, which came with the pair yet to face a court hearing, was secured as Australia returned scientist Reza Dehbashi to Iran. He had been detained for 13 months in Australia over the purchase of a defence system from the United States for use in Iran.

While pleased to hear of Ms King and Mr Firkin's return to Australia, Mrs Zaghari-Ratcliffe has been left to question why British officials have not yet secured her release, according to her husband Richard Ratcliffe, who telephones his wife regularly.

"I told Nazanin yesterday and she was very pleased that Jolie was released," Mr Ratcliffe told The Times.

"Obviously for Nazanin it is also bittersweet. While there is no queue for cases like this, she did ask how come the Australians can solve their hostage cases in a couple of months while the UK has taken over three and a half years. I had no answer."

Iran's foreign minister Mohammed Javad Zarif last month said Ms Zaghari-Ratcliffe's fate was connected to a historic debt Britain owed Iran over a decades-old weapons deal.

Mrs Zaghari-Ratcliffe and her husband recently decided their five-year-old daughter Gabrielle would be returned to the UK, having been in Iran since her mother's arrest in 2016.

Meanwhile, another British-Australian woman, Kylie Moore-Gilbert, is still in Evin prison where she has been for almost a year.

Dr Moore-Gilbert, a Cambridge-educated, Melbourne-based academic specialising in Islamic and Middle East studies, has reportedly been handed a 10-year sentence, also on a charge of spying.

Australian officials are still working to secure her release.