The talks in Vienna are aimed at securing by July 20 a mammoth deal to reduce the programme and ease fears the Islamic republic will get atomic weapons.
Iran denies wanting the bomb and wants punishing UN and Western sanctions lifted.
"It has been another really tough round," said a diplomat from one of the "P5+1" six powers -- the five permanent members of the UN Security Council plus Germany.
"That doesn't surprise me or particularly dismay me since from the very beginning we have always known that if a deal was to be done, it was going to be very difficult," the envoy said.
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"The talks are being held in a serious and productive atmosphere, but progress in drafting the comprehensive agreement has been limited," one Iranian diplomat told the IRNA news agency.
The trickiest issue is uranium enrichment, a process making nuclear fuel for civilian purposes but also, when highly purified, for a nuclear weapon.
Western countries want Iran to slash the number of centrifuge enrichment machines in order to make it harder for Iran to process enough material for a bomb -- if it chose to -- in a short period of time.
"We are absolutely focused on July 20 ... We are not interesting in talking about a rollover," the P5+1 diplomat said, adding it would be a "long time" until such an extension is even discussed.
IRNA reported that the next round of talks would take place from July 2 in the Austrian capital. There was no official confirmation on this from either side.
Mark Fitzpatrick, a former US State Department official now at the International Institute for Strategic Studies in London, said it was "not surprising" that difficult topics were being put off until later.