Human rights in Iran have worsened under President Hassan Rouhani despite his reformist agenda, but Tehran may take steps to improve its record if a nuclear deal is concluded, a UN investigator has said.

More than 200 people have been executed in Iran already this year, in addition to 753 known executions last year, the highest annual figure since 2002, said Ahmed Shaheed, UN special rapporteur on human rights in Iran. About half of the executions are for drug offences.

He was speaking in Geneva as US Secretary of State John Kerry and Iranian foreign minister Mohammad Javad Zarif held talks on Iran's disputed nuclear programme in the Swiss city of Lausanne to try to narrow gaps before a March 31 deadline for a political agreement.

"In terms of overall impact of the negotiations, there is a lot of concern amongst the Iranian society that the nuclear file may be casting a shadow over the human rights discussion. My view however is that if there is improvement in terms of the engagement on security issues, there is potential for improvement in other areas as engagement widens," Mr Shaheed said.

Mr Rouhani has failed to create a freer society since his landslide election on a progressive platform in 2013 but has urged patience in bringing about liberalisation, something resisted by the powerful security services and judiciary.

"In my view the overall situation has worsened as indicated by the surge in executions and ... other legislation in place which would further undermine the space for the NGO community, the space for the legal community, the space for political parties and for human rights defenders," Mr Shaheed said.

The former foreign minister of the Maldives has not been allowed to visit Iran since taking up the independent post but has interviewed people still in the Islamic Republic as well as exiles in Western Europe.