The US and Iran say they are confident of reaching a deal over Tehran's nuclear programme after agreeing a seven-month extension to talks.
Iranian President Hassan Rouhani said a deal was close but US Secretary of State John Kerry warned that several points of disagreement remained.
Six world powers want Iran to curb its nuclear programme in return for the lifting of economic sanctions.
Tehran says it is not seeking nuclear weapons, but wants atomic energy.
The six countries - the US, UK, Russia, China, France and Germany - have been in negotiations with Iran to finalise a preliminary deal reached last year in Geneva.
They have now agreed to extend discussions, with the aim of reaching a high-level political agreement by 1 March, and confirming the full technical details of the agreement by 1 July.
Iran would be allowed to continue accessing $700m (£450m) a month in frozen assets during that period.
Speaking in Tehran on Monday evening, President Rouhani said the gap between the two sides had "narrowed" during the latest round of talks in Vienna.
"It is true that we could not reach an agreement but we can still say that big steps have been taken," he said in an interview broadcast on state television.
Mr Kerry was more cautious, warning that although "substantial progress" had been made there are still "some significant points of disagreement".
"These talks are not going to get easier just because we extend them. They're tough. They've been tough. And they're going to stay tough," he told reporters in Austria's capital.
Analysis: Jeremy Bowen, BBC News, Vienna
All sides here in Vienna wanted a deal. None of them wanted to walk away from a negotiation they accept is difficult. The reason is that the alternative to a deal might in the end turn out to be war.
Before the initial agreement in Geneva a year ago the Middle East seemed to be sliding, slowly but inexorably, into a war over Iran's nuclear plans.
Israel had threatened an attack so many times that the threats were starting to lose credibility. But they still had to be taken seriously. And Israel still views Iran as a great threat. In a BBC interview while the negotiators were talking in Vienna, Israel's Prime Minister Netanyahu once again compared the Islamic Republic of Iran to Nazi Germany.
The deal they've been discussing in Vienna is complex, but technical details are not the main reason why they need more time to talk. At the heart of the talks there still is no agreement on the vital equation - the amount of uranium that Iran would be able to enrich, and the extent to which sanctions against it would be lifted.
In a joint statement, Iranian Foreign Minister Mohammad Javad Zarif and EU envoy Catherine Ashton said that based on "the new ideas which continue to be explored" they are convinced a "comprehensive solution can be reached".
"We intend to build on the current momentum in order to complete these negotiations within the shortest possible time," the statement read.
Although Mr Kerry declined to comment on the details of the negotiations, President Rouhani said the talks had shown there was "no doubt for anyone that enrichment will continue in Iran".
The remaining sticking points are thought to be:
- The future size and scope of Iran's capacity for uranium enrichment work
- How quickly sanctions on Iran are lifted
- Unexplained explosives tests in Iran that could be linked to a nuclear weapons programme.
Speaking to the BBC, UK Foreign Secretary Philip Hammond said the failure to agree a deal was "a disappointment, but rather than continue blindly we have to recognise the reality that we're not going to make a deal tonight".
Under the terms of international treaties, countries have the right to develop nuclear energy, which Iran insists is its only aim.
However, the International Atomic Energy Agency (IAEA) says it has been unable to confirm that Iran's activities are exclusively for peaceful purposes.
Highly enriched uranium can be used to make a nuclear bomb, but uranium enriched to lower levels can be used for energy purposes.
The UN Security Council has adopted six resolutions since 2006 requiring Iran to stop enriching uranium, with sanctions to persuade Iran to comply.
The US and EU have imposed additional sanctions on Iranian oil exports and banks since 2012, hitting Iran's oil revenue badly.
24 Nov 2013: Six-month interim deal agreed; aim to reach final deal by 20 July 2014
19 Jul 2014: Interim deal extended until 24 Nov
24 Nov: Agree to meet again in December and extend deadline for "political agreement" to 1 March 2015 and final deal by end June 2015
Both Saudi Arabia and Israel are vehemently opposed to Iran acquiring nuclear weapons.
In the US, three influential Republic senators, John McCain, Lindsey Graham and Kelly Ayott, called on President Barack Obama to "tighten the economic vice on Tehran" through new sanctions.