Vienna on Tehran's nuclear programme and expect to meet again soon to resume their work, the European Union said on Friday.
"Reflecting the complexity of the technical issues discussed, it became clear that further work is needed," saidMichael Mann, spokesman for European Union foreign policy chief Catherine Ashton.
He said there would be consultations in capitals and talks were expected to continue soon.
Experts from Iran and the six powers talked for four days to try to work out exactly how to implement last month's breakthrough deal for Tehran to curb its nuclear programme in return for limited sanctions easing.