Iranian Foreign Minister Mohammad Javad Zarif was to leave Vienna and return to Tehran for consultations with the country's leadership on the state of negotiations, Iran said.

Officials from both sides said big differences still remained.

European Union foreign policy chief Frederica Mogherini said earlier on Sunday it was not impossible to get an accord by the self-imposed deadline but that a few extra days may be needed. 

Foreign ministers from the negotiating countries were gathering in Vienna on Sunday to assess where the talks stood. 

They expected them to spill over into July.