agreed for the resumption of talks searching for an elusive breakthrough after more than a decade of tensions.
The world powers had offered some easing of the sanctions that have hurt Iran's economy in the past two years, in return for Tehran accepting limits on its nuclear programme.
But speaking after the talks wrapped up, Ashton admitted that the world powers were still waiting to see "real engagement" from Iran over the proposal.
"It became clear that the positions (of the world powers) and Iran remain far apart on the substance," Ashton told reporters.
"It's pretty obvious what we want to achieve in the end and we have very often spelled that out. The challenge is to get real engagement so that we can move forward with this."
Russian deputy foreign minister Sergei Ryabkov said the two sides had failed to find "mutual understanding", and that a time and place for the next talks remained to be agreed, in comments quoted by Russian news agencies.
Ashton said the sides had decided to go back to their capitals to discuss what to do next and promised that she would be in touch with chief Iranian nuclear negotiator Saeed Jalili.
Ashton represents the world powers - comprised of the five permanent UN Security Council members and Germany and known collectively as the P5+1 - as chief negotiator.
Iran's team is led by Jalili, a hardline conservative close to Iran's supreme leader Ayatollah Ali Khamenei. Jalili, who has until now held out little hope of concessions from Tehran, is due to hold his own press conference later.
The clock is running down on diplomacy to solve the crisis, with Iran's archfoe Israel refusing to rule out air strikes against Iran to halt its nuclear drive.
As well as bilateral discussions, the two sides held a plenary meeting in the morning, then continued in the afternoon. Another plenary session took place in the evening, officials said.
The two sides had held an indecisive first day of talks on Friday that ended with only an agreement to meet again and Western officials admitting the positions were still far apart.
Western officials had expressed puzzlement at Iran's reaction after the first day, saying it had not put forward clear and concrete proposals.
Ashton started Saturday by meeting Jalili in the hope of establishing whether Tehran's position had shifted from its tough demands of Friday.
Iran insists on international recognition of what it says is its "right" to enrich uranium, a key component of the nuclear fuel cycle which can also be used to make the explosive core of an atomic bomb.
But the world powers insist on Tehran ending enrichment to high levels and verifiably suspending operations at the Fordo mountain bunker where such activity takes place before recognising Iran's rights to less threatening nuclear activities.
Iran has reportedly been offered the right to deal in some precious metals and perform small financial transactions now prohibited by the international sanctions. Tehran says it is being asked too much for too little in return.
The P5+1 grouping is especially worried about Iran's enrichment to levels of up to 20% and wants the Islamic republic to ship out the part of its 20-percent enriched uranium not converted into powder for reactor fuel.
Iran denies it is developing the atomic bomb and argues that it requires a nuclear programme solely for peaceful medical and energy needs.
However it has repeatedly stalled in talks with the UN nuclear watchdog which wants expanded inspections of Iran's programme to address suspicions of military-related work.