AFP/ Geneva

Iran hopes the case of an American journalist jailed and charged in the country can be resolved, but it is a matter for the courts, its foreign minister insisted on Wednesday.

Jason Rezaian, a Washington Post bureau chief in Tehran who holds dual US-Iranian citizenship, was charged in early December after a lengthy court appearance.

But the specific accusations against him remain unclear, according to the Post, and it is not known when he will next appear in court.

"The government is doing its best to be of assistance. This is a judicial matter," Iranian Foreign Minister Mohammad Javad Zarif told reporters as he waited to meet his American counterpart John Kerry in a Geneva hotel for nuclear talks.

"We will have to wait for the judiciary to move forward, but we try to provide all we can in assistance."

Rezaian, 38, was arrested in July with his wife, Yeganeh Salehi, who was freed on bail in October. In December, Iranian authorities said Rezaian's detention would be extended for up to 60 days.

Kerry has said he was "distressed" at how Rezaian's case has been handled, and slammed Iran for flouting the law by denying him access to a lawyer.

He called on the Iranian authorities to drop the charges "and release him immediately."

"The United States is deeply disappointed and concerned by reports that the Iranian judiciary has charged Washington Post journalist Jason Rezaian with unspecified charges, and that the judge denied his request to be released on bail," Kerry said in December.

But Zarif did not appear to hold out any hopes for an early release for Rezaian, insisting Wednesday: "We hope that this issue can be resolved. But unfortunately there are issues involved -- judicial issues involved -- which the judiciary has to deal with."

Zarif also made a clear distinction with the case of US Marine Amir Hekmati, who has been imprisoned in the Islamic republic since 2011.

"He has an Iranian passport," Zarif said, adding that nationals with "dual citizenship" were subject to the laws of "the country of origin".

Arrested in August 2011, Iranian authorities convicted Hekmati of spying for America's Central Intelligence Agency.

He was initially sentenced to death in 2012 but Iran's top court subsequently reduced the penalty to 10 years in prison.

In late December, the United States on Tuesday denied reports it had offered Iran a prisoner swap to retrieve Hekmati in return for the freeing of Iranians in the United States