Boris Johnson has called for “progress” on the cases of British citizens detained in Tehran as he invited Iranian president Hassan Rouhani to London for talks.
The Prime Minister met with the Middle Eastern country’s president at the UN General Assembly in New York amid heightening tensions with the Islamic republic.
Mr Johnson had blamed Tehran for the recent attacks on Saudi Arabian oil facilities, branded the Iran nuclear pact a “bad deal” and was facing calls to speed up talks over British citizens being held in the country.
A number of British-Iranian dual nationals are being held in Tehran on highly-disputed allegations, including mother Nazanin Zaghari-Ratcliffe.
Mr Johnson invited Mr Rouhani to London "where we can discuss things more" as the two held talks at the summit on Tuesday.
"But there are other issues also I wish to raise with you and we retain serious concerns about the detention of dual nationals in Tehran and we are looking forward to make progress on that," he added.
The PM said his visit to Tehran in 2017 as foreign secretary was "very productive but so far inconclusive", adding: "I think we still have a lot of progress to make."
Ms Zaghari-Ratcliffe is more than half-way through a five-year sentence over spying charges, allegations she vehemently denies.
Her husband, Richard Ratcliffe, had called on the PM to tell Mr Rouhani that "enough is enough" and that he must free the mother immediately.
Mr Johnson's meeting with the Iranian leader came two days after he formally accused Tehran of being responsible for the Aramco attacks, which forced up oil prices.
The attribution of responsibility - which Tehran disputes - earned him a swift rebuke from Iran.
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The PM went on to criticise the Joint Comprehensive Plan of Action (JCPoA) as having "many defects", in a break from previous UK policy.
But he went on to reaffirm British commitment to the JCPoA in a joint statement with France and Germany, as they blamed Iran for the Saudi attacks in a display of unity.
The PM's meeting came immediately after Mr Rouhani met with French President Emmanuel Macron, and the three spoke briefly in the same room in the Millennium Hilton hotel opposite the UN.
Mr Macron said it would be a "lost opportunity" if US President Donald Trump does not meet Mr Rouhani before he leaves the states.
Addressing Mr Rouhani, Mr Johnson said: "I think I agree with Emmanuel. You need to be on the side of the swimming pool and jump at the same time."
