Iran nuclear talks deadline extended to end of June
The deadline for a nuclear deal with Iran has been extended to the end of June after talks in Vienna failed to reach a comprehensive agreement.
US Secretary of State John Kerry said that the talks had been "tough", but "substantial progress" had been made. The parties will reconvene in December.
Iranian leader Hassan Rouhani said most gaps had "narrowed" during the talks.
Six world powers want Iran to curb its nuclear programme in return for the lifting of 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 after the Vienna talks had ended, Mr Kerry said: "We are certainly not going to sit at the negotiating table forever.
"But given how far we have come over the past year, particularly in the past few days, this is certainly not the time to get up and walk away."
Analysis: Jeremy Bowen, BBC Middle East editor, 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.
Mr Kerry declined to comment on the details of the Vienna negotiations, arguing that confidentiality was needed to allow free discussions.
However, he said that "progress was indeed made on some of the most vexing challenges that we face".
Meanwhile, Iranian state TV quoted President Rouhani as saying: "Our positions with the other side got closer."
He said there was a time when world powers were entirely against uranium enrichment in Iran, "but today, there is no doubt for anyone that enrichment will continue in Iran, and our nuclear technology will continue".
There are thought to have been three key sticking points in the Vienna negotiations:
- 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 Mr Hammond said that 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 Tehran's nuclear 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.
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
The United Nations 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.
Both Saudi Arabia and Israel are also vehemently opposed to Iran acquiring nuclear weapons.
Israeli Prime Minister Benjamin Netanyahu, whose country has not ruled out bombing Iran's nuclear facilities, told the BBC that "no deal is better than a bad deal".