The European Union says more work is needed before world powers can implement last month's interim nuclear deal with Iran.
Officials from the so-called P5+1 countries and Iran met for four days this week in Vienna to negotiate the details of the landmark deal, which aims to curb Iran's nuclear program in exchange for sanctions relief.
A spokesman for EU foreign policy chief Catherine Ashton on Friday called the talks "lengthy and detailed." But he said it became clear that more work is needed because of the "complexity of the technical issues discussed." He did not elaborate.
The spokesman said there will now be "consultations in capitals, in the expectation that technical talks will continue soon" on turning the deal into a concrete action plan.
The deal reached on November 24 is seen as a precursor to a comprehensive agreement aimed at addressing Western concerns that Iran is trying to create nuclear weapons, something Tehran denies it is trying to do.
Officials from Iran and the United States, France, Germany, Britain, China and Russia met at the Vienna headquarters of the U.N. nuclear agency, which will play a central role in verifying that Tehran carries out its part of the interim deal.
The outcome of the meetings were expected to determine when Iran stops its most sensitive nuclear activity and when it gets the respite in sanctions it has been promised in return.
Under the deal, Tehran committed to limit uranium enrichment for six months and not to make further advances at its Fordo, Natanz and Arak facilities. Iran will also neutralize its stockpile of medium-enriched uranium - which is easy to convert to weapons-grade - and allow more intrusive IAEA inspections.