Talks with Iran over releasing British-Iranian Nazanin Zaghari-Ratcliffe are "going right up to the wire", Boris Johnson has said.
But the Prime Minister added he could not say more as "negotiations (with Iran) continue to be under way".
Asked by broadcasters at the Emirates Palace hotel in Abu Dhabi whether a UK negotiating team was currently in Iran capital Tehran, the Prime Minister said: "It is true and it has been for a long time that we're negotiating for the release of our dual nationals in Tehran.
"There are some very sad cases, including Nazanin.
"I really don't think I should say much more, I'm sorry, although things are moving forward.
"I shouldn't really say much more right now just because those negotiations continue to be under way and we're going right up to the wire."
Mrs Zaghari-Ratcliffe has been detained in Iran for nearly six years after being arrested while taking her daughter to see her family. She was accused of plotting to overthrow the Iranian government, which she denied.
Yesterday it emerged she has been given her passport back in what supporters hoped would be a major breakthrough.
She was reportedly told by Iranian authorities that she was being detained because of the UK's failure to pay an outstanding £400million debt to Iran.
Foreign Secretary Liz Truss today insisted the debt was a “separate issue” to detained dual nationals - but said the UK was seeking to pay it.
She told Sky News: "We are working very hard.
"I’ve made it a priority as Foreign Secretary to secure the release of Nazanin, Anoosheh [Ashoori] and Morad [Tahbaz] who are all detained in Iran.
"We‘ve also made it a priority to pay the debt we owe to Iran, but I can’t say anything more than that.
“We have been clear this is a legitimate debt that we do owe Iran and we have been seeking ways to pay it.”
Asked if the debt has already been paid she replied: “I can’t say anything more about that at this moment.”
Labour's Tulip Siddiq said Nazanin Zaghari-Ratcliffe has been talking about coming back to the UK as "the day that she's been dreaming about for six years now".
The MP told BBC Breakfast: "We know that there is a British negotiating team in Iran, and it's difficult to think why they would be there if there wasn't some leeway in what was happening, that there may be some progress made on the case. Certainly Richard is feeling hopeful.
"I also was in touch with Nazanin as well, who definitely sounds a bit more stressed and a bit more nervous than Richard does. But at the same time is talking about coming home, being reunited with her husband and her daughter, being back at home in West Hampstead and saying that this is the day that she's been dreaming about for six years now."
At the end of last year, Mrs Zaghari-Ratcliffe's husband Richard went on hunger strike outside the Foreign Office in London.
Richard Ratcliffe has said that the government "only deals with problems when they become crises", and has been heavily critical of its response to the family's plight.
He called on Prime Minister Boris Johnson to acknowledge that his wife is being held "hostage" in Iran, and has pitched a tent in central London where he plans to keep a vigil outside the government building.
Mr Ratcliff ended his hunger strike after 21 days.
At the time he said his job was to "keep going", adding: "We probably hoped we'd get a breakthrough doing this. We haven't yet.
"I didn't want to go out in an ambulance. I want to walk out with my head held high."
In January, the daughter of another British-Iranian detained in Iran said her father was to begin a hunger strike due to a lack of progress in securing his release.
Retired civil engineer Anoosheh Ashoori has been held at Evin Prison on charges of spying for Israel, which he denies, for more than four years.
In a video posted in relation to the hunger strike, Elika Ashoori said she was at that time "extremely concerned" for her father's health "as he approaches his 68th birthday".
