Prime Minister Theresa May has used telephone talks with Iranian president Hassan Rouhani to call for British prisoners held in the country to be released on humanitarian grounds.
Mrs May also used the dialogue to restate Britain's commitment to upholding the Iran nuclear deal, but condemned Tehran's missile attacks against Israeli forces.
The PM brought up the issue of British dual nationals being held on charges in Iran, like imprisoned mother Nazanin Zaghari-Ratcliffe.
A Downing Street spokesman said: "The Prime Minister raised the issue of consular cases relating to dual nationals currently held on charges in Iran and called for further progress to be made so individuals could be released on humanitarian grounds."
Mother of one Mrs Zaghari-Ratcliffe has been in prison in Iran for more than two years.
On the Iran nuclear deal, which US president Donald Trump abandoned last week, Mrs May said it was in "both the UK and Iran's national security interests" to maintain the agreement, according to the spokesman.
The PM welcomed Mr Rouhani's public commitment to abide by its terms, and said it is "essential" that Iran continues to meet its obligations.
The spokesman added: "The Prime Minister made clear that the UK condemns the Iranian missile attacks against Israeli forces and called on Iran to refrain from any further attacks. She said it was important to avoid provocative actions to ensure peace and security in the region."
Both leaders "agreed the importance of continued dialogue" between the two countries, and looked forward to the meeting of UK, German, French and Iranian foreign ministers in Brussels on Tuesday to discuss the Iran nuclear deal and next steps.