Iran nuclear: Zarif says powers 'changing positions'
Iran's foreign minister has accused world powers of making it harder to reach agreement at nuclear talks in Vienna by taking different positions.
Mohammad Javad Zarif said "several countries" were also making "excessive demands". His UK counterpart said the process was proving "painfully slow".
The so-called P5+1 - US, UK, Russia, France, China and Germany - said talks would go on until at least Monday.
The powers suspect Iran of seeking nuclear weapons, which Iran denies.
Iran says its nuclear programme is for purely peaceful purposes.
The sides have been holding marathon negotiations to reach a long-term agreement and have missed successive self-imposed deadlines.
Both Mr Zarif and US Secretary of State John Kerry said though that they would not be rushed into making a deal.
"We're here because we believe we are making real progress," John Kerry told reporters on Thursday.
But he cautioned that "we are not going to sit at the negotiating table forever" and that he was prepared to "call an end" to nuclear talks with Iran if "tough decisions" are not made."
The failure to reach a deal by Friday means the US Congress will now take twice as long - 60 days - to decide whether to accept or reject any eventual deal, keeping sanctions in place until then.
Mr Zarif said late on Thursday that the negotiating position of the P5+1 was making it harder to reach a deal.
"Unfortunately we have seen changes in the position and excessive demands... by several countries," he said.
The sides have continued talking long past a self-imposed 30 June deadline to turn a framework deal into a comprehensive, final agreement.
The P5+1 and Iran are working on a deal which will curb Iran's nuclear programme in return for sanctions relief, though the pace of the lifting of sanctions has been a major sticking point.
Analysis: BBC's Lyse Doucet, Vienna
Another deadline was missed - but no side in this marathon is calling it a bad sign. Negotiators emphasise how far they have come, how close they are.
But at this 11th hour there are, unsurprisingly, conflicting views on the impasse. Western diplomats speak of a small number of political and technical issues to resolve and place the onus on Iran.
But a senior Iranian official lamented that Iran now feels it is in five bilateral negotiations since every country has different "red lines." A senior Iranian official would only speak "of certain changes in the US position" as well as "a walking back" by other countries - they deny that.
It is clear more time is needed - but how much is not clear to anyone. An Iranian official said this wasn't "the 11th hour, but 13th or 14th". This is a process which, from the start, has had a clock all its own.
The P5+1 and Iran are still believed to differ in three key areas - international inspections of Iran's non-nuclear sites, sanctions relief, and how Iran's compliance will be verified.
Iran also wants a UN Security Council arms embargo to be scrapped - something the US has ruled out.