Foreign Secretary Boris Johnson has held "frank" talks with his Iranian counterpart in Tehran over the case of imprisoned Briton Nazanin Zaghari-Ratcliffe.

The two-hour meeting with Iranian foreign minister Mohammed Javad Zarif covered a number of areas, including Ms Zaghari-Ratcliffe's situation.

A Foreign Office spokesman said: "They discussed the full range of bilateral issues and they both spoke frankly about the obstacles in the relationship, including the Foreign Secretary's concerns about the consular cases of British-Iranian dual nationals.

"Both emphasised their commitment to continuing to work together to improve the bilateral relationship."

The spokesman added that the meeting had been "constructive".

"During two hours of frank talks, they agreed to work together to make progress on a number of the most important bilateral and regional security issues.

"The Foreign Secretary began by expressing his condolences for the powerful November earthquake which struck Iran's border region with Iraq.

"He then moved on to the nuclear deal, underlining the UK's continued support for the deal and determination that it should be fully implemented.

"They also talked at length about the breadth of regional issues and the need for regional partners and the international community to work together to find solutions.

"Overall, it was a useful meeting at the start of the Foreign Secretary's visit to Iran."

Mr Johnson has been seeking the release of Ms Zaghari-Ratcliffe who is serving a five-year sentence over allegations, which she denies, of plotting to overthrow the Tehran government.

Ms Zaghari-Ratcliffe was arrested in 2016 during a holiday visit to show her baby daughter Gabriella to her parents.

Reports suggest she could appear in court again on Sunday, following threats to increase her sentence by five years following Mr Johnson's gaffe of telling a parliamentary committee that she had been in Iran to train journalists. He later acknowledged this was not the case.

Her husband, Richard Ratcliffe, said he was "waiting on tenterhooks, biting my nails" ahead of Mr Johnson's visit to Iran.

"I'm really pleased he is there in time for Nazanin's trial and waiting to see what will happen. I'm certainly hopeful but I'm trying not to be expectant," he told The Guardian.

It is understood that Mr Ratcliffe did not accompany Mr Johnson on his trip after receiving advice that such a move may not help his chances of seeing his wife in prison.

"If I'm blunt, it is better that he is there in time for her trial than he and I go there after her trial and she's been sentenced to more years," he told the newspaper.

When the couple last spoke by telephone on Tuesday, Ms Zaghari-Ratcliffe was "genuinely worried about the court case and getting quite agitated", her husband said.

Mr Ratcliffe told the newspaper that Mr Johnson will attempt to visit his wife in jail and meet the head of the Iranian judiciary.

However, Tehran does not recognise Mrs Zaghari-Ratcliffe's dual UK-Iranian nationality, and refuses access to her for representatives of the British authorities, making a prison visit for the Foreign Secretary unlikely during the trip.

Ms Zaghari-Ratcliffe's case is one of a small number of cases of dual nationals whose release Britain is seeking on humanitarian grounds on which Mr Johnson will push for progress.

The Foreign Office has declined to name the other individuals involved - or even identify the number in jail - after their families asked for their cases to be kept out of the public eye.

Mr Johnson's trip to Tehran is only the third by a UK foreign secretary since 2003 and comes at a time of tension in the Middle East over Donald Trump's announcement that he is recognising Jerusalem as the capital of Israel.