over Iran's nuclear ambitions in the Kazakh city of Almaty but the agreement on new meetings suggested potential for progress.
The talks saw the five UN Security Council members and Germany offer Iran a softening of non-oil or financial sector-related sanctions in exchange for concessions over Tehran's sensitive uranium enrichment operations.
A senior US official said Iran "appeared to listen carefully to the offer" and its chief negotiator Saeed Jalili issued rare praise for the world powers' "positive" and "realistic" attitude.
Speaking in Vienna, Iran's foreign minister Ali Akbar Salehi went even further, saying he was "very optimistic about the outcome".
"Things are taking a turning point and I think the Almaty meeting will be (seen as) a milestone," Salehi said.
Jalili, seen as close to Iran's supreme leader Ayatollah Ali Khamenei, was more circumspect, saying the world powers' proposals were "more realistic, compared to what they said in the past".
"We consider these talks as a positive step which could be completed by taking a positive and constructive approach and taking reciprocal steps," he told reporters in Almaty.
Uranium enrichment is the most sensitive part of the nuclear cycle as the process can be used to make both nuclear fuel and the explosive core of a nuclear bomb that the powers fear Iran wants to develop.
Officials said the sides would next meet at the level of senior civil servants on March 17-18 in Istanbul.
Talks involving Jalili and the six world powers represented by EU foreign policy chief Catherine Ashton would then take place in Almaty on April 5-6.
'We need to see confidence'
In contrast to the more effusive Iranian response, Ashton refused to be drawn into a judgement of the talks' success.
"I hope that the Iranian side are looking positively on the proposals we put forward," Ashton told reporters.
"The proposals we put forward are designed to build in confidence and enable us to move forward.
"We approach this with the absolutely united view that we need to see international confidence in this (Iranian nuclear) programme."
The US official added that the meeting had been "useful".
"I wouldn't say it was positive or negative."
The offer reportedly involves easing sanctions on Iran's gold and precious metals trade and lifting some very small banking operations.
In return, it demands a tougher weapons inspection regime and the interruption of enrichment operations at the feared Fordo bunker facility where 20-percent enrichment goes on.
"This is interesting because what we are seeing is the start of a process," said Moscow-based PIR nuclear safety research institute analyst Andrei Baklitsky.
"The positions are slowly starting to merge. In other words, there are finally things there for them to discuss."
Iran, however, has always countered that its right to enrich uranium must be respected before negotiations can proceed.
The US official said this right is explicitly ruled out by UN Security Council sanctions punishing Iran for failing to cooperate with nuclear inspectors.
"There is a cost for Iran for every day we wait to solve the problem," the senior official said.
Iran has also stipulated that it would only consider giving up enrichment to 20% if all forms of sanctions against it were lifted -- a condition unpalatable to Washington.
The 20% level nears the "red line" degree of enrichment required for weapons-grade uranium.
US Secretary of State John Kerry said on a visit to Berlin on Tuesday that he hoped "Iran itself will make its choice to move down the path of a diplomatic solution".
Israel -- the Middle East's sole if undeclared nuclear weapons power -- has never ruled out attacking Iran's nuclear sites and the diplomacy is essentially aimed at avoiding such an outcome which would send shock waves across the region.
Later Wednesday, Israel's Prime Minister Benjamin Netanyahu said Iran will face "military sanction" if it does not halt its nuclear program.
"Iran is continuing to defy the international community," he said.
"Like North Korea, it continues to defy all the international standards and I believe that this requires the international community to ratchet up its sanctions and make clear that if this continues there will be also a credible military sanction," he said.
The Kazakhstan talks also come as US lawmakers on Wednesday planneed to introduce a bill that would tighten sanctions no Iran.
The bipartisan House legislation, a copy of which was obtained by AFP, would allow US President Barack Obama to impose penalties on foreign entities that provide Iran with goods that help maintain its struggling economy.