AFP, TEHRAN: Iran said on Friday there were still "some major problems" in finalising a nuclear agreement with world powers as the clock ticks down to a Tuesday deadline.
"Some major problems exist which are still blocking the work... but in other areas we have made good progress," Deputy Foreign Minister Abbas Araghchi told state television from Vienna where he has been negotiating with the six powers.
"Overall, the work is moving ahead slowly and with difficulty," he added, without going into details.
"Extending the deadline is not on the agenda... but if necessary we will work for a few extra days to reach a conclusion."
US Secretary of State John Kerry was due in Vienna on Friday to join the drive to fill in the holes in a framework accord struck in April to seal a historic deal.
Iranian Foreign Minister Mohammad Javad Zarif is to join him on Saturday as Tehran strives to secure an end to years of isolation and crippling Western economic sanctions in return for reining in its nuclear programme.
Iran's official IRNA news agency quoted a source close to its negotiating team as saying there were few gaps to be filled to finalise an agreement but that they were big ones.
"It is true that progress has been made and that the gaps have been filled in a large part of the text of a final agreement," the source said.
"But the gaps that remain all involve issues of substantial and essential divergence."