Iran's foreign minister has said that a detained Washington Post reporter with dual citizenship is facing interrogation in Tehran for what he has done as an Iranian citizen but has expressed hope for his release.

Mohammad Javad Zarif, on a visit to New York for the latest round of nuclear talks taking place on the sidelines of the UN general assembly, told National Public Radio (NPR) that he considered Jason Rezaian, an Iranian-American, as a fair reporter but had little information about his current situation.

Rezian was detained on 22 July after security forces raided his home in the Iranian capital and arrested him, his wife, Yeganeh Salehi, also a journalist, and two other Americans. The pair remain in a Tehran jail nearly two months after being arrested.

Zarif said Tehran was treating the journalist, who has dual American and Iranian citizenship, solely as Iranian.

"Whatever he has done, and I'm not in a position, nor do I have information to share with you about what his charges are, but whatever he has done, he has done as an Iranian citizen, not as an American citizen," he said, according to a transcript of the interview. "And he is facing interrogation in Iran for what he has done as an Iranian citizen."

Iran does not recognise dual citizenships, meaning that those with an additional nationality detained in the country on political grounds are in a vulnerable situation and would not have consular access.

Iran's intelligence and security apparatus, in particular, have deep suspicion of dual citizens and have arrested a number of them in recent years for crimes that human rights activists have alleged to be politically motivated.

Rezaian's arrest, which has caused much embarrassment to President Hassan Rouhani's administration, highlights an internal rivalry between his moderate faction and the hardliners. After being elected, Rouhani went to the Washington Post to publish his first and only opinion piece, in which he outlined his goals for a moderate future.

Zarif said he knew Rezaian personally and expressed hope he would be released soon.

"I hope that all detainees will be released. I believe that it is in the interest of everybody to work for a more positive atmosphere. And that's what I've done in the past several months," he said. "But I believe that people have to face justice, if they committed crimes. Of course if he didn't commit any crimes as an Iranian citizen, then it is our obligation as the government of Iran to seek his release."

Despite his prolonged detention, little information has been released about Rezaian's current situation or the charges he is facing.

His detention is believed to have been carried out by the country's hardline judiciary, which operates independently of Rouhani's government. That means Rouhani's hands are tied but as the president of the Islamic republic, he can use his position to influence other political institutions in the country.

Zarif said Iran's judiciary had no obligation to provide any updates to the US about Rezaian's situation in jail.

"The judiciary has no obligation to explain to the United States why it is detaining one of its citizens … His lawyers know. He knows his charge," he said. "And nobody's waterboarding him."

However, Rouhani's inaction over a series of arrests in recent months, including that of Rezaian, has exposed the limits of his power.

The International Campaign for Human Rights in Iran (ICHRI), a New York-based non-profit organisation, said there was a disconnect between the Iranian president's conciliatory rhetoric on domestic affairs and the continued detention of dozens of prisoners of conscience in the country.

"Rouhani states, 'I have not forgotten anything; I am standing by all my positions.' But the people of Iran need more than words; they need action," said ICHRI's Hadi Ghaemi.

"Rouhani states, 'We want change and transformation.' But change does not fall from the sky. He must show leadership and enact change. He may claim that as president he has limited authority, but he has not yet exhausted all his powers," he said.

"Rouhani has put all his eggs in the foreign policy basket, but that has also made him vulnerable. Opponents of his foreign policy are emboldened by their ability to perpetuate domestic repression and will sooner or later undermine his foreign policy too."