The husband of Nazanin Zaghari-Ratcliffe has told Sky News he is "not very confident" of a breakthrough in her case and it "may have a while to run still".
Richard Ratcliffe had a 10-minute phone call with Liz Truss, the new foreign secretary on Sunday and told her that his wife's case should be the top priority and he wanted to see the government tackle hostage-taking head on.
Mr Ratcliffe told Kay Burley that his wife feels "more reassured" after his call with the foreign secretary, but expressed his view that the case may not be resolved in the near future.
Ms Truss has said she will push for UK nationals trapped in Iran to be released, during a meeting with Hossein Amir-Abdollahian at the UN General Assembly in New York on Monday.
She is expected to bring up the cases of Ms Zaghari-Ratcliffe, Anoosheh Ashoori and Morad Tahbaz.
Ms Truss said: "I will be asking Iran to ensure the immediate and permanent release of all arbitrarily detained British nationals in Iran, and to begin working with us to mend our fractured relations."
She had earlier spoken to Mr Ratcliffe while at the airport before leaving for New York, and is said to have shared his concerns about the situation.
Mr Ratcliffe's wife, a British-Iranian dual national, has been in custody in Iran since 2016 after being accused of plotting to overthrow the government.
Mr Ratcliffe told Kay Burley: "I was very pleased to be able to speak to her [Ms Truss] yesterday.
"In all honesty I don't think she knows quite how it's going to go and certainly from here I don't think we know either.
"There's been lots of foreign secretaries that have met with the Iranians, lots of negotiations have been happening, particularly in the last year.
"I spoke to the former foreign secretary [Dominic Raab] a month ago where he said 'listen, we got really close but we didn't get it over the line'.
"I think there's been a change of leadership in Iran as well.
"We need to see where things are and whether things have kind of where they were, in which case it could be quite hopeful, or whether they've slipped back a long way.
"We've had a lot of ups and downs, so not very confident... preparing as to whether we need to go tougher or not, and over the weekend we sent in a request to the Foreign Office to sanction some Iranians involved in hostage-taking.
"Clearly we are feeling this may have a while to run still."
Mr Ratcliffe earlier said that he had given Ms Truss the names of 10 people he accuses of being involved with "hostage-taking" in Iran and added in his Sky News interview that Ms Truss "needs to make sure that Iran doesn't carry on practicing hostage diplomacy".
Foreign Office minister James Cleverly told Sky News the release of dual nationals by Iran will be "top of the agenda" when Ms Truss meets her Iranian counterpart.
"We will continue pushing and we will not rest until we get them all home," he told Kay Burley.
"Their incarceration is arbitrary, it's completely unjustified and we have made it clear that we will not stop working to get those British dual nationals home."