Iran said on Friday there were still "some major problems" in finalizing 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 led by the United States.
"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."
U.S. 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 program.
Iran's official IRNA news agency quoted a source close to its negotiating team as saying there were few gaps to be filled to finalize 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."
The news agency did not detail the points of contention, but other Iranian media said that they concerned the timetable for the lifting of sanctions, access for U.N. inspectors to Iranian military sites, the long-term future of Iran's nuclear program, and the scale of its future research.
Iran and the six powers already missed two deadlines in their quest for the outline deal reached in Switzerland in April.
A senior U.S. official also acknowledged on Thursday that negotiators may miss next week's deadline as they thrash out the final elements of a deal.
"We may not make June 30, but we will be close," the official said.
The mooted final accord between Iran and the six powers — Britain, China, France, Russia and the United States plus Germany — will be a highly complex agreement some 40-to-50 pages long, and including several appendices.
It will set out an exact timetable of sanctions relief and reciprocal steps by Iran, as well as a mechanism for handling possible violations by either side.
Tricky issues include how U.N. sanctions might be re-applied, the reduction of Iran's uranium stockpile, and its future research and development of newer, faster types of centrifuges for uranium enrichment.
Iran must also address lingering questions about the possible past military dimensions of its nuclear program to the satisfaction of the U.N. watchdog agency.